Wow Jennifer - I'm sorry to hear you are having this problem. I bet it is a bit unnerving, not to mention odd given you were fitted.

Speaking of fitted, obviously you were, but I've always felt that the dynamics of riding indoors (whether on rollers or a fluid trainer) change the variables a bit. When you ride outside I think you tend to move around much more for reasons that are readily apparent - whether the usual stopping/starting to changing your line to going uphill/downhill.

I know this may seem like a no-brainer but when you first get this feeling, have you tried moving your toes and arm around? Sort of the same thing you do to shake off the foot falling asleep syndrome? When I do really long climbs (say 10+ miles) I have a tendency to start feeling a little numbness in my hands - so wiggling my fingers a bit, plus not gripping the bars so tightly gets rid of the numb feeling quickly.

I hope some of this helps, or that one of us can come up with useful ideas for you.

Hang in there, it will be better