My horse is sweating on her hind legs for what appears to be no reason?

I'm trying to figure out if I should be concerned about this weird sweating or not. I have a 4 year old Thoroughbred mare, just got her about 3 weeks ago. She stays out all the time, except to feed when she comes in to her stall. I usually take this time to groom her. I've noticed she is sweaty on her hind legs, from about her hocks up. Its concentrated close to her rear, and she is warmer to the touch on these areas. Her chest is not warmer than normal, and she is not sweating anywhere else. She isn't lame and is eating and behaving normally. It isn't hot outside (about 60 with a nice breeze) And she hasn't been worked. This has happened at least 4 times now, probably more. Does anyone have any insight to what might be going on??