I used to feel that way a lot back in the day before we started doing accelerated deco with O2 - fatigue an hour or two after the dive, and after a 3 day weekend of two deco dives per day, I'd get almost flu like symptoms Monday morning. The fact that O2 made me feel so much better post dive clued me in that there was more going on here than just "tired".
I tend to take the information the computer gives me as advisory, and if I feel a little fatigued post dive, it's an indication I need to get out a little cleaner and I'll scale back the GFs and add some extra time to the O2 stops (10 and 20 ft). On most light to moderate deco dives I'll pad the deco about 5 minutes total, adding a minute or two to the 20 ft stop and the rest to the 10 ft stop. The gas switch evolution and time needed to go on O2 at 20 ft, naturally slows the ascent from 30 ft to 20 ft and I use an intentionally slower than normal ascent from 20 ft to the 10 ft stop, with a very slow ascent from 10 ft to the surface.
We also do a fair amount of off gassing on the surface post dive, about 5-10 minutes before we get out of the water and at least 15-20 minutes before we start hauling the heavy gear. It there are steps, hills, etc, I'll walk out, remove the wing and harness, get un suited, dry off, etc, then go back and haul the tanks last.
It's pretty conservtive, but the end result is I feel great post dive with no fatigue. We do 2-3 dives a day, with most of them being deco dives, on our usual 7 dive day trips and the extra stop time and post dive off gassing plan does not leave me feeling worn out at the end of the week.
