I had this problem too. Somewhere I read that if you use toothpaste with tartar control, it could cause those bumps. I used a different toothpaste and the bumps went away.
Hi
This may be allergic contact dermatitis or folliculitis. Do you shave this area? If you do, is folliculitis likely? Do you have any known allergies? Do you apply any lotions or creams on the area? Any allergies to certain food ?
You may need to have this assessed by your dermatologist. You may need to apply topical medications for this. Topical steroids may be able to help but with close supervision from your doctor. If this is folliculitis then topical antibiotics may be necessary.