3 month old sucking fists after feeds?

Tobias is 3 months and for the past couple of weeks he's sucking his fists a lot. He does it when he's hungry, but he also does it just after feeds... however he will fall asleep doing it (which wouldn't happen if he was ravenous). Lately it seems he's doing it all the time and makes me think he's hungry all the time! I don't remember my first doing this... but he had a pacifier and this one doesn't. Is it a hunger thing or a "wow-I've-got-hands" thing?