Has/does this happened to anyone?

For the most part I'm emotionally rather flat, but every so often it's like the dam safeguarding me from intense and painful emotions bursts. Triggered by something I've heard or read.It's impossible to know in advance what it'll be. I feel like I've been thrown into a deep pit of despair.

Yes I have had the same thing happen and I do think it's a micro trauma, certain words and actions that seem so small can be so shocking and vexatious to the soul, I think context matters too, if someone refers to my dog as a ***, I grit my teeth as I don't like it but I know its the correct word for a female dog especially in certain circles. If I'm walking down the street and pass someone who muttes it at me then it feels much more serious and shocking.

While I am not sure I have the ideal vocabulary (for myself) for something similar to what you are describing, I believe I know what you mean. 

The best way I can describe it is: something catches you unawares (you had not anticipated it), maybe to you it seems really out of place but it has instantly transported you to a thought or sensation from another context or point of reference, it seems something like some kind of micro-trauma?  Probably only something you will have noticed when you are among other people.

You might not always understand quite why and doubtless those around you will not have noticed anything themselves and are unlikely to have noticed your mini-stall (so no need to worry about that consideration). 

It can feel a bit like something you had not expected just sort of stops you in your tracks for a moment.  It is not necessarily limited to a word trigger. 

It does not always have to feel like a dramatic shock or startling experience - other times it just may seem like a realisation which you really did not need to have as an interrupt your train of thought / actions underway at the time.  

When it occurs, I try to think to myself along the lines of: "Oh, that was a thing, although it was a surprise - I don't need to always know why right now, notice it, maybe have a think about it later if it still bothers me then, it is fine to continue with whatever I am doing, nobody noticed, reset and resume (everything is fine - you can relax about it)". 

It sounds like you have cPTSD (complex post traumatic stress disorder) which is incredibly common amongst autists.

Some have major events that caused it but most have an accumulation of smaller events stemming from rejection or overload from their autistic traits that cause it.

There is a good article explaining this here:

https://www.webmd.com/mental-health/what-are-ptsd-triggers