brandi... have you had your bike fitted professionally? considering you have pain only on the one side, it makes me wonder if you have one leg that is slightly shorter than the other... which would mean you would need to shim the cleat of the short leg...

typically pain in the back of the leg indicates too high of a saddle, but if the other leg is ok...? it's difficult with something like this to pinpoint it without seeing you ride... but your butt should not be so sore that you need a heating pad... definitely not normal!

yikes... hopefully someone else can help more... here's a link on fitting...

http://www.peterwhitecycles.com/fitting.htm

here's a link to knee pain caused from cycling.. including when it hurts behind the knee...

http://www.cptips.com/knee.htm

hope you can get it fixed soon... nothing worse than hurting from doing what ya love!