Daytime sleeping

I wonder if anyone here has experienced this. Just putting out that my LO has been eating on schedule, gaining appropriate weight and length, and the doctor says everything is fine with him, BUT...
As of yesterday, his daytime sleeping has been really strange. He used to take long naps during the day and have two, two-three hour awake periods. The past two days, he's been practically awake the whole morning and afternoon. We wake up around 7:00, make breakfast, play, etc... Then around 12:00, he starts getting really tired and fussy. I try many times to put him down for a nap, but he won't stay asleep until the evening (3:00 or 4:00) when he FINALLY goes down for a nap. Are our long nap days over already? Lol