What Causes A Blister In Between The Buttocks?

My mother is 87 and has recently had a bout with a uti then double pneumonia. She sits, pretty much all of the time, in her recliner. She was complaining of her butt hurting so I looked. Just inside her upper butt crack is a hole that resembles a burst blister. It s smaller than a dime, and it looks clean , but it still concerns me. Any suggestions as to what may have caused this, and what do we do to treat it?