Summer,
My first thought when I read your post title was that the questions should be "Does therapy ever NOT exhaust you?"
Between getting keyed up before hand and then how intense a session can get, I am usually pretty wrung out by the end of a session. I am VERY grateful that I have a job that allows me to telecommute and work at home, so after really intense sessions (I usually see my T early in the day) I just head home.
My first T was really helpful with this, she used to tell me that a really intense session was like you had just ran a marathon. She was really big on the need for self-care, knowing what a drain it could be to do this work.
The truth is that emotions are how we move energy through our nervous systems. When you process and express long-buried, unprocessed feelings, you are moving an enormous amount of energy and it's a lot of really hard work. I sometimes think of it as the way you get tired on a long car trip. Yes, you're sitting, but your body is forced to make constant physiological adjustments to handle the motion, that when you arrive, you can be really wiped. Therapy is like that. Just because it's not physically visible, doesn't mean you not exerting a tremendous amount of effort. I have had sessions (had one on Wednesday actually) where I can literally feel stunned by the end.
Near the end of the session, I said to my T "I'm so tired" and he pointed out something I had never thought of that could be a factor also. He talked about tiredness being a dissociative reaction. Which made sense because I had spent a large chunk of the session fighting through dissociation because of the level of material I hit.
Which is all my really long-winded way of saying, you're probably working very hard and using a lot of energy. And yes, I've taken days off from work at times. My favorite excuse if I didn't want to be up front about it, was a 24-hour stomach bug. Everyone knows they completely lay you out for 24 hours, but you do recover quickly from them.
My other favorite excuse was a really bad sinus headache. I have a lot of problems with my sinuses due to allergies, so it always sounded plausible.
AG