I know exactly what you mean!
My DS (who is 5.5 months old) has been fab at sleeping during the day - and then the last three days he seems to have decided that he doesnt need them!
 
 
  he had one sleep today for 2 hours (he fell asleep on my lap) - but i'm wondering if maybe he is having some teeth movement going on?
He was quite grizzly

  and just wasnt himself so i gave him some pamol and a rusk to have a little chew on and that's when he decided to have a bit of a sleep.  
Anyway...am sending sympathy vibes your way - here's hoping both our DS's settle back into some kind of routine soon!