Like someone else said, from an old/current injury is probably the case... The other possibility could be that since she hasn't had much done with her, she isn't carrying herself correctly. It's hard to assess without seeing the horse go, do you have a trainer or someone knowledgable that could watch her go for you? If she is traveling crooked (as every horse does, no horse is naturally straight, especially one that has had time off) then she may feel slightly "off", but not lame. If it is coming from the hind legs, try lots of loosening exercises to get her pushing from both hind legs (such as leg yeilds, serpentines at the trot, lots of bend changes and ask her to really step under herself with that leg). If it is an injury to the hamstring (as someone else said), you will notice her getting worse. If she is maybe a little sore or stiff from just sitting around, she will get better.
Hope that helps! :)
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."