If you're referring to the hip flexor, that pain will be in the front where the leg crease is. It could be anything from a fibrous tendon sheath getting irritated (hence the bursitis), a shortened muscle group (knee injury) pulling on the bony attachments, the bursa itself getting further irritated, the hip capsule itself getting pissy, all the way to what the other posters have mentioned.
Some stretching of the hip flexor is indicated (there are many stretches out there - just don't over arch your back while doing them). Hold your stretches for at least a minute for plastic remodelling, do several times a day but don't stretch in pain. Back off if you feel pain and hold where it feels 'tight'.
Physiotherapy, massage, hellerwork, rolfing are all good ways to reduce the fibrous adhesions which you likely have. Stretch your quads, too, since they contribute to the hip flexion. HTH.