Tired signs

My son is 4 month old (19 weeks nearly at the end of leap 4 thank God!!!) Recently he hasn't been showing me any tired signs.. this morning I thought he was tired put him down in his crib and he was lay awake in there for over an hour chatting and laughing I knew he was tired.. I had to give up in the end as I had to pop to town and low and behold he fell straight asleep in his pram after 3 hours of being awake. When we come home again showing no tired signs he has a little play and then a bottle.. he falls asleep on the bottle. I don't understand. He normally can't last any longer than 2 hours awake. But more recently especially first thing he will go alot longer.. not sure if he's fighting it as come afternoon he will go off after less than 2 hours. I'm so confused by it all and I get so stressed by it it can make me cry sometimes as I just want my baby to sleep ok. Night times are fine.. he goes to bed between 8 and half 8 and goes to sleep on his own after 10-15 mins. (He does wake several times at night once for a bottle and a few times for his dummy but I can deal with that) Its just day times and naps that are the problem as he doesn't show me he's tired anymore!? :/ Help!?