How To Treat Rashes On Buttock?

I got a rash on right buttock, I got it the next day from sitting on the toliet at work I normally won't do that but it was an emergency. The rash has been there a year now, it comes and goes in the same spot. When it's flared up it itches and it feels like a lump, like a aint bit when I touch it, it's sore and red, trigged by fabrics lotions and certain soaps. When not flared there's no lump just the mark it leaves, kind of light brown, I'm white complexed