Hey ! Awkward question lol….

I have this rash on my bum and the inside of the cheeks that is almost like a “diaper rash” , it is cold where I live so it’s not a heat rash. I don’t know when to see a doctor or treat it myself. I tried an oatmeal bath last night, it seemed to help at the time but it is currently really bad. I got my boyfriend to look and he said it looks very painful and looks like it’s almost going to bleed. Now I’m currently on my period and wearing a pad and it’s super irritating. I don’t wear tampons. This started before my period came. I’ve had this for about a week now but recently started to get bad a few days ago. Any recommendations for treatments or what it could be , please let me know.