Bottle teats

Hi mummies who bottle feed, I’ve been using Pigeon 0+ months Twin pack (comes with teat SS size) to bottle feed my baby with ebm since birth. He’s 2mo now and recently been crying and struggling when feeding. Idk why and am wondering if it’s becos the milk flow has become too slow for him and he’s frustrated. Should I change the teat to S size now? When do youall usually first upsize the teat?