Breastfeeding question

I breastfeed on demand and usually he eats 25 to 30 minutes each side and he will be full for 2 hours. But the other day I didn't breastfeed for that whole day I didn't even pump and at end of the day I fed baby until he was full and I pumped the rest to empty out. Ok so that was 3 days ago. Ever since then I've noticed he's been more fussy or maybe it's normal. But today i had him on my breast for 2 hours and he never got full. I thought he was but when I took him off he went crazy crying and showing signs of hunger. I fed him for another hour and still hungry and fussy. So I gave him formula. Could my milk supply be low ??