Ulcer on butt

Hey so I saw a few people asking about ulcers they found on there vag but I think I have one on my butt crack near by anus. I noticed this like 4 days ago but it started feeling like a bump so I thought it was a pimple and over time it kinda got uncomfortable when I was sitting or if my underwear gave me a wedgie and I pull it out it would hurt. Yesterday I went to touch it to see if I could pop the “pimple” and I felt like a dent where the pimple was. I checked down there and saw like a small yellowish white spot. The skin around it is red. Idk what it is. Help???