Whatever amount you were using, I would probably just use slightly less for awhile and see if it stops happening. Then if you want to find out the absolute maximum she'll take, you start increasing very slowly while watching for signs of stress again. Once you see the first sign of stress you know that is too much, and you need to use the amount just before the one that caused stress.

Or if you are monitoring your PPM, if you have say, 1400, try 1325 instead.