I think it was an ingrown hair originally. It became more like a spot, burst and then won't heal. When I think it has, it then bleeds again and is sore.
I've currently shaved the area in case the hair was preventing it from healing, and have put sudacrem on. I think the area, close to the leg crease, means it gets bothered a lot by normal leg movement when walking and sitting, so maybe that's why it is very slow (months now) to heal.