Horse with a stifle issue?

Okay, I have been riding a friend's Thoroughbred mare for about 3 wks. She has explained to me that she had a stifle issue awhile back. I've been riding her, getting used to her, and she's a great mare, just been spoiled and not been ridden for about 2 yrs. She's gotten a bit barn sour and buddy sour though. Anyways, she'll walk and stop fine, but whether I'm trotting or loping her, she has a hard time stopping when I ask her to. So when I try to turn her in circles to make her stop, they start out real big, and then smaller until she finally stops. And there's no problems with the bit because she has a soft mouth. Also, I've been training her for barrels, so when I ask her to lope, she hesitates and pins her ears back and slows down. So I'm wondering whether these issues have to do with her stifle? Or is the loping issue just her in heat and all marish and moody? I'd love some tips and help. Thanks, your help is much appreciated [: