The famed Therapy Hangover. You didn't mention what therapy you are doing but all of them even "talk therapy" have their price to pay afterword.
Learning what affects you most lets you plan a little for the days after therapy. You can't prepare for everything but you can start learning the skills you need in your toolbox to deal with those days. For me - it's not adding stress for a start, so eating, drinking, self care, rest (even if it isn't sleep) and its also grounding, and things to help stay away from rumination.
One of the big ones is learning to pace your therapy. Mine is EMDR and talk. So we haven't done EMDR for a couple months because we (I refer to my T and I as we) did some heavy work on anger and I know the signs of reprocessing work for myself. Piling more EMDR work on will make me far less functional right now so in the meantime we do talk therapy. I find it really helpful because I get "PTSD Blind" and can't see things that seem very obvious sometimes.
But it also helps to spread out EMDR or other therapy work because the problem most people get into and don't realize is they go charging along in therapy until "PTSD Strikes Back" and they end up nearly non functioning and overwhelmed. Any therapy takes its own time you can't just push and rush through it. My T said this last week that's where most of her patients get into trouble. You push PTSD, and it shoves back.....