Regression at the age of 4+

Hi

My son is 4 years old &autistic.

He regressed at around 11 months, which is quite common. However, I've noticed several recent regressions, the strangest is swimming, which he really enjoys. 

However recently, for no known reason (i.e. he has not had a bad experience) he is scared to go in the water, and gets upset, its almost as though he's forgotten he enjoys it...

I'm wondering if others have experienced anything similar as I'm just looking to try to understand the switch, I've tried looking at books etc. but regression at his age isn't well documented. 

Thanks in advance.