TMI butt....

So this is a weird question/topic. So let me just say before getting pregnant I was very skinny. No butt, no hips nothing. Well I have gained quite a bit (I'm 24 weeks now) and now my butt (along with my legs, belly, and boobs) is much bigger. So here's where my concern is.. and its weird... so I'm just gonna say it... my upper butt crack feels like its splitting open (like my tailbone)!!! It is painful and stings! It happened one time before just not as bad but I had just got back from a long road trip and thought it was from sitting in the car for 14 hours. Has this happened to any one else? I feel weird bringing it up to my midwife but since its now happened twice I'm not sure if it's normal?