I think this lump is a sebaceous cyst. Nothing to worry about but see a DR if it gets painful
I actually just went to the doctor for the same thing, and she told me it was a clogged pheromone gland, and would resolve itself naturally.
I believe it is a harmless cyst. Most women have then and don't even know it. The can be anywhere from breasts to hands. They are normally painless, harmless, knot type lumps, but they can "flare up" swelling and becoming painful but are never a cause for concern unless that cause a lot of pain, then a doctor can remove it. I have one in my breast and they said that unless it hurts more often than not there is no sense in removing it, leaving a scar and causing pain to cut it open.
Sounds like a lymph gland. These glands swell and hurt when you have an infection in the abdomen or on your leg. Should have one on the other side also. - If not, see your doctor - it might be a lipoma. (fatty cyst)