Why does she keep stopping half way through?

Everytime I am on top she will make me stop after two minutes to catch her breath then ask me to go on top again, when im on top her hips are bucking like a bronco, but when i ask her she says she just gets tired but breathes heavily is she not enjoying it or it because I'm a lot bigger than her and she can't handle my weigh for too long. We tried with her onto but she doesn't llike that