What could i have if i found a tender lump under my skin between my groin and thigh?

A couple of things: The most common being an enlarged inflammed lymph node - so called inguinal lymph node. It could also be an inflammed epidermal inclusion cyst, or an early abscess - organized infectious collection. Ideally you should seek the input of your doctor to make sure this isn't something that will require treatment sooner rather than later or isn't a signs of a more worrisome process going on.