You're not a baby or a freak!! Your tailbone may be bruised, geez, we all get achy in areas when we aren't used to doing certain exercise... Wait till Wednesday, take a break of a few days even, if it's that sore, then get back in the saddle
un intended: and try again... Eventually you'll be fine. I am always sore the first few times on my bike in the spring....
The seat may not be the right seat for you in that class... I have an ergonomically correct seat for my bike, I love it ... but I still get a bit sore the first few times out!!
Sitting in a nice hot bath may help, get a massage, take ibuprophen like youi said...