It could be an enlarged lymph node - we have a lot of lymph nodes in the groin area - especially around the crease as you describe.
Many slim/skinny women and men can feel them easier than a heavier person.
My sister had this and thought it was cancer - doctor confirmed it was just her lymph nodes - she is very skinny.
I had this too - and didn't notice a change in size, no pain - so presumed it was just a lymph node.
If it changes in size or becomes tender/painful - visit a doctor.
i had this, i just ignored it because all the google i did on it said it might be an ingrown hair but its still there in the back of my mind wondering what it really is,,,i see my doctor regulary since im pregnant and hes never said anything about it. if you do see your doctor and get a different diagnose. let me know!