It is hard to provide very specific advice, because it is not totally clear from your question where exactly the lump is and whether you are male or female. For that reason, I'd recommend visiting your primary care doctor
, so that a medical professional can take a look at the lump and give you a detailed opinion.
In both men and women, small lumps on the skin around the genitals could be from simple infections of the skin - such as a boil - or an ingrown hair. Both of these tend to produce a lump that is reddened and tender to the touch. Simple skin infections or ingrown hairs usual only require good skin hygiene, but more serious cases may require drainage or antibiotics.
If the lump is in the crease between the abdomen and the thigh, then it might be a lymph node. Lymph nodes in this region could be enlarged for several reasons, but the first order of business would be ruling out an infection, which could include a sexually transmitted disease, as the glands tend to enlarge when they are fighting off an infection.
The final general possibility would be a cyst, which would be a form of a growth under the skin.
Call and make an appointment with your doctor!