6 month old suddenly sleeping LOADS!

My little boy is 6 (almost 7) months old and for the past 3 or so weeks he has suddenly become really really sleepy during the day. He's sleeping for like 2 hours straight and then is only awake for about an hour then is so sleepy and cranky again until he finally then falls asleep again! He is usually very active and playful and very smiley and never cries unless he's hungry. At the moment he's crying, angry, nothing makes him happy and he's just not himself at all. He hasn't cut any teeth yet so I'm wondering if it's just teething or a growth spurt or something, but it's been going on for weeks! He doesn't have a temp and he doesn't seem poorly. He is constipated though as he takes gaviscon for reflux. Any help or advice ladies...?!