Hard pimple like bump where undies meet my groin. not in genital area, more to the side. only one and it hurts sometimes. what could it be?

Several things: It sounds like you might have a simple cyst or folliculitis Hard to say without a physical exam. Apply warm compresses 3-4 times a day for 20 minutes a time for a few days as it may help with swelling and pain. If that does not help, I think you will need to see your Doctor in order to have an exam and determine a diagnosis.

An infection: You are describing an infection or a small abscess probably an infected hair follicle. See your doctor for antibiotics and possible drainage of the abscess.