If you can, get a friend or yard staff (or if you have time, then you!!) to bring her in a few hours before you ride to let her "deflate" her grass belly a bit before you get on to ride her! :) If not, a grazing muzzle sounds like a good idea.

Could you fence her field off so she doesnt have too much to eat? Or get her a grazing muzzle. This used to be a problem with my gelding, but now hes paddocked hes just as forward and responsive in the summer as in the winter. She probally just feels bloated and a little but urgh. Imagine running round after eating a HUGE roast dinner!