Why has my 11 month old switched his nap schedule?

my 11 month old boy usually is like clockwork with his nap schedule. the past 2 days he has stopped that schedule and has decided to go longer without naps. today he has only taken 1 nap and it is 5 pm. we woke up at 8. the nap also only lasted 30 minutes. yesterday from noon to eight he didn't take a nap at all and then just crashed out for the night. has anyone else had this issue?