You may have a tight flexor muscle (psoas muscle). This muscle flexes your torsal foreward and helps flex your thigh. When you bend forward you take the tension off but when you straighten it tries to stretch. You may also have trigger points in this muscle. This muscle is attached to your thigh bone, pelvic, and the transverse process of your lower spine. Some of its referred pain pattern is the abdomen, low back, groin, and thigh. Seek the help of a sport massage therapist or an ART practitioner (Active Release Techniques). I wish you well in your pursuit to wellness.
Ray Mingo CMT, PT