Core strength?

All winter your bike was in the trainer and steady. All your core had to do was hold you up.

When you rode outside for the first time, suddenly your core had to hold you up AND deal with the movement of the bike. (like it had in the Fall)

Maybe your core got a bit out of practice after being on a nice stable trainer stand for a few months.

A few more rides, and it should strengthen back up and get coordinated again.