Rash after swimming

EVERYTIME I go swimming I always get a painful red rash on my bum cheeks where my bathing suit sits. It’s itchy, very tender. I’ve never been to the doctor for it but everytime I swim I’ve gotten it for the past couple years. I use hydrocortisone cream and it eventually goes away in a couple days but always goes really dry and flaky first.

Anyone know what this rash is??