Why when i sit down my butt starts to get hot & i fell very uncomfortable?!

Its started in like the 9th grade and happens at times now. I'm afraid of sitting down next to someone cause i might start fidgeting all around and keep getting up, they might think i'm weird. The problem really is that i become paranoid thinking that my butt is hot and so it smells, which i don't think as it literally starts when i start to think about it and i've asked one persom before but idk if it smells other times even though I dont smell anything but i really dont know. Its always in the back of my mind which causing it to get worse. I can usually sit down comfortably but as soon as i start to think " I hope it doesn't start, it slowly starts. Whats going on? Has this happened to anyone else and how to stop it?