If your performance is dropping in the gym, you may need to increase calories a bit.
I have seen (and experienced it myself) setting
PR's while dieting. It's a matter of not getting into the mentality that you have to "feel the burn in order to burn fat." Stick with heavier loads, keep the diet clean, and maybe toss in a few days per week of brisk walking.
But again, my first hunch is that you dropped total kcal intake too low.