Hey stranger,
I actually asked my OB about this in terms of sleeping on back & she said that there was no reason not to!
She said if you are comfortable, then do it and there was no danger to you or baby to do it, my GP said the same.
I have to admit I am still a bit nervous as ALL the books say not to!
Good on you for going to the gym 4 times a week!
I am down to twice.
My ob said the same thing.

You will know when to stop as it becomes 'uncomfortable'. For me I felt extreme pressure near my fair bone which would start to travel down my leg. This started around 24 weeks but only became really noticeable at 28.