Ours is also on Zantac and she's a completely different baby. She only fusses now when she is hungry or wants some attention. Before the medicine, our nights were terrible with screaming for a couple hours. And she was very fussy during the day. She now plays by herself and doesn't constantly need us there. It literally only took one or two days to see a difference in her.
She's only on .6 ML twice a day and she weighs more than your baby. I'm surprised our pediatrician doesn't have us give her more.