Dramatic regression - any experience?

Hello all, am a first time poster. My daughter turned 4 years old about 6 weeks ago. Just prior to this, her excellent pre-school told us they thought she needed a Special Needs Assessment. Her Dad and I were initially quite shocked as she always seemed developmentally ‘normal’, if not advanced! They mentioned things like her motor skills (can’t ride a trike, hates slides) and lack of interest in the other kids - but also said her speech and language were ‘amazing’ and her imagination was wonderful. They said she appeared to be interested own world, loved to line things up, didn’t want to do any work. I knew they were implying ASD straight away. To be honest we weren’t convinced, we thought that as an only child who doesn’t know a single child outside of nursery, we had mollycoddled her (always putting her coat on for her etc) and with a bit of work on this and some play dates she would be fine. She also struggles to write her name - she gets distracted and draws all over the page! But she can do it with a real effort and direction, and we thought as a leftie she was just taking her sweet time! 

After some reading I realised she did display some traits - although she was super chatty and interested in the world, she would struggle with a reciprocal conversation - ie. she would just tell you about her favourite stories / cartoons and you would be oh so interested! But she wouldn’t ask about your day or thoughts, although she did have a good sense of humour and would actively mock us! She was only 3 after all. She would make up songs and stories, and liked to play alone a lot, but she was role playing and chatting to her toys while doing so. She spent lots of time with adults and was so well behaved with good manners, most of the time! But was awful at sharing with her baby cousin and would literally push her away. She held her poo in a lot when she was little (sorry tmi!) but we had got over this with a star chart and rewards. We started to come to terms that she probably had ASD but considered it to be relatively unobtrusive and at the milder end of the scale. Bear in mind she was still only 3 at this time.

THEN a week after her 4th birthday, she went in for a routine operation on her eyes (squint surgery) - we were told it was affecting her ability to concentrate and could be causing her headaches, and so we consented to the op. She has had a terrible time since. Long story short, she had 2 full weeks of sheer hell - hallucinations, psychosis, terror, extreme distress and screaming - we took her to A&E, the doctors, spoke to the anaesthetists...she was off her rocker and it was the most distressing thing I’ve ever seen in my life. My poor child. 

This faded away and she would still talk about crazy stuff - but more nonchalantly. It is being investigated what happened but it’s possible she had a bad reaction to one of the drugs she was given. Then she had a period of being near catatonic, silent but also unreachable - she couldn’t understand what we were saying, and couldn’t give a *** anyway. She would burst into hot tears suddenly, and be unable to tell me why. I could not go to work this entire time, and she stopped speaking almost entirely. She then became very babyish, needing constant cuddles and love, wanting to be spoon fed, acting inappropriately, her language disappeared completely, which is so shocking as she was so good before. She stopped eating and lost weight. 

Now she is having good days and bad days - emotionally she seems generally happier, however she has almost completely stopped speaking. She can go 4 days at a stretch with no words. Even when she does speak, it is very basic now, ‘yes’ ‘no’ and if you tell her ‘I love you’ she might say it back - if we instruct her to say good morning, she might on a good day. Very little independent speech, and no more imaginative play or bringing things to our attention. We had thought she had trauma, PTSD or similar - but she is now displaying so many odd behaviours we have never seen before - her facial expressions and the way she moves her body have completely changed. She looks and acts differently. She stares at lights, pulls odd faces, paces up and down a lot, growls, screams, has angry outbursts, holding her poo in again - she is like a different girl. The house is so quiet now, we are so sad.

The paediatrician we saw thinks she has regressive autism, and is investigating this. We are working hard to keep her occupied and happy and she does say the odd thing now. She seems to have regressed such a lot, it is really dramatic. However she sleeps perfectly well thought the night again now and eats like a horse again (she has an amazing diet and eats lots of varied foods, is a vegetarian.) 

I suppose what I wanted to ask was a few things - does anyone have experience of a regression like this? It is so dramatic and as I say her facial expressions are different, and she is older than the average onset of 19 months for regression. Can she regain the skills she has lost, or are they gone forever? And does anyone have experience of this happening/ adverse reaction to general anaesthetic? The paed. described it as a neurotoxin and said it could have exacerbated things. I can’t stop crying on her bad days ( not in front of her) - I miss my girl and her chatter so much. I am due to have another baby in the next 10 days or so and am so worried this will make her worse, too. Any experience or advice please people, we are desperate over here. SORRY for epic post! x