This is Sam almost every day. Seriously. It's gotten worse since we've been trying to eliminate bottles in favor of the straw cup during the day.
But I agree with you - totally think it's that "nap hangover" feeling. The best I've been able to do is to give him some time alone (5-10 min) to wake up, then we rock a little or read a book, and snuggle. Then go downstairs and have a snack (making sure it's fruit or something he likes a lot). By then he's usually doing better. Not fun though!
We are on vacation now and it really does screw up their sleep schedules! DD used to have a morning nap around 930 am but now she is staying up til 11-1130am and still fighting a nap. I think we may be transitioning to one nap soon but I still feel like its kind of early so I try to do two naps but bed time gets pushed way late. I have noticed though that now that she goes down at 8-9pm, she sleeps through the night so I'll take it! I am sure it will be all over the place when we get home but that always happens with vacations.
We have "nap hangover" sometimes too....usually, it happens when DD is hungry...once you give her some food, it seems to perk her back up and she's her happy-go-lucky self again
Re: Angry after afternoon nap...
BFP #1 12/02/11, M/C 12/08/11
BFP #2 04/06/12, DD born 12/20/12
BFP #3 06/09/14, M/C 06/15/14