Cyst possibly. Heat packs on it/warm baths. they are pretty painful after being messed with. Women especially get them a lot down there ... all of the moisture and friction.. very very common. I can't comment on the medical "m" term as "M" is not enough for me to go off of in guessing.
After you talkin about how long its been there and its only a bump UNDER the skin... i'm 95% positive it is not herpes. Herpes bumps and blisters are ON not UNDER the skin. They have a very stuck ON appearance. Like... an arts and crafts eyeball piece with a sticky substance on the back. You stick it on the paper. Think of your skin as the paper and a herpes bump or blister as the eye.
just don't mess with it ok, it could cause greater irritation or cause a secondary infection of some sort.
Oh, and try not to touch any other bump that appears down there, in case some day you do end up getting the hsv virus you do not want to touch it and then spread it all over the body. Bad bad deal! And also you and your partner you two could go get type specific ( IgG )blood tests (herpeselect) and have them check ya'll for hsv 1 and hsv 2. Knowing your status is always a good thing.
Or if you can't resist messing with the bumps (Shaving, folliculitis, or otherwise), cover any cuts on your fingers or palms and hands and then take them off after and wash really really well with warm soapy water to kill anything that could be on them (bacteria, possible virus particles and just germs period).
lots of good warm packs and soaking! :)