Sounds like a hamstring injury, but you really should have it checked out by a Dr. If it's a tear, you MUST stay away from gymnastics until it heals... if you keep reinjuring it, you will be out of the sport even longer. In the long run, it's much better and easier to have an injury checked out righ away and just deal with it!
My poor DD has suffered a broken collarbone, severely sprained ankle, and not an overstretched ligament in her knee.
The ankle injuiry is the one where she and I kept ignoring it, hoping it would get better, and it finallly ended up keeping her out for more than 4 months. If we had just tkaen care of it when it happened, it would've been a short Physical therapy issue and she would've been back in about 2 months. LESSON LEARNED.