When you sleep is when your body rebuilds itself and repairs muscle tissue, so if you are not used to exercising that much, it could be that your body just needs more sleep.
You could try scheduling a nap during the day, going to bed earlier and/or getting up later so that you are getting the sleep you need but it is not interfering with your schedule.
Also, do you go straight from exercising to no activity at all? Do you go straight from the gym to plopping yourself down on the comfy couch. You should try to remain moderately active - take a walk (fresh air is good) do some light housecleaning/straightening, etc., so you don't have a sudden drop in adrenalin.