Why can't I walk up the stairs?

Tonight we met with our wonderful trainer. She killed us! The pain is a great feeling but at the same time it isn't. I know that it is paying off. Basically, she changed our whole workout which is nice because I was starting to get tired of the old one. I think that the stuff that we are doing now is showing how much we have improved since we started. So, I come home from the gym and start to go upstairs except my muscles hurt so bad. So, I take one stair at a time and finally make it ha ha! After showering and what not I go back downstairs to watch some TV. I sat there for about an hour and then attempted to get up and realized that I am going to be extremely sore come tomorrow. I have to suck it up. I know this is good for me and I know that this is going to help me reach my goal. Today food wise, was pretty good. I didn't get as much fruits and vegetables in as I would have liked or should have, but did not go over my calorie limit so I am happy with that. I am hoping that I am able to get out of bed tomorrow morning to go to spinning, I will try my hardest!