@ElizaBennett - Just call an HVAC person. You can take some time first to observe the system and see if it seems to be running more often than usual, not blowing as cold, etc., but really only a pro can tell you what's wrong.
I've had this problem, the high AC bill, and it's been totally different problems. One time, our condenser was running constantly instead of shutting down when not in use. (Probably should have noticed that sooner. Oops!)
Another time, the heat was sort of stuck on and it was blowing at the same time as the AC. Not sure how that could work with the reversing valve in place, but somehow it was.
Unless you yourself have special training, you pretty much always need to call a pro for your heat pump, but reading articles like this one can at least help you fine-tune your BS detector for evaluating what the person tells you!