I am in naptime hell-- HELP

DS has always been fairly easy to put down for naps, we do a naptime routine then I lay him down awake and he falls asleep within 10 minutes.  For the past four days he has been fighting me for his naps and screaming bloody murder the second I put him in the crib.  I usually end up letting him cry for 30 minutes until I can't take it anymore then I give up and get him. If I bring him downstairs figuring he is not tired, he is clingy and cranky. 

So the last 4 days in a row he has taken ONE 30-40 minute nap a day and therefore is waking once in the middle of the night (he used to sleep straight through)  I know its probaby a phase, but did anyone else have this happen?

I dont know if he is transitioning to one nap a day or what, but I also found that even trying to hold him off later for his nap he will still fight me and cry.  I'm about to pull my hair out.  His bday party is this weekend, and I'm hoping he will nap for me so that he is not a cranky mess.