growth spurt?

My LO is five weeks old. Last week we upped him from 4 oz to 5 as he just never seemed full. Well the last couple nights he has been grunting and stretching out a lot while he is sleeping.He has acid reflux and was doing this prior to getting him on medicine.We also had to try a couple different kind of formulas. He has been on Similac Alimentum for a week and has been doing fantastic on it. I didn't think too much of the grunting at the time but today he's been fussy through out the day and grunting a lot. Tonight right after his 5 o'clock feeding he started getting very fussy and stretching and grunting like he does while he's sleeping (also sometimes while he's stretching, his legs will twitch, almost like a tremor - I'm not sure if that's normal or not). Well now I'm starting to wonder if maybe he is hitting his six week growth spurt (a week early)  and may need more to eat or if has something to do with the acid reflux since he was showing similar signs before he started the zantac. His pedi had said not give him more then 5 oz but that was last Mon and at that time was two days short of 4 weeks old. I should also point out that he is a pretty big boy weighing 12.3 pounds at that appointment. So, like I said, I don't know if he is just hitting this growth spurt I keep hearing about and he needs more to eat or if it's related to his acid reflux. I'm just afraid to give him too much to eat if he doesn't need it but I also don't want him to go hungry. Any help would be appreciated,