my baby isnt gaining weight


so initially my baby girl would spit up after mostly every feed and still managed to gain around 800gms, whereas in the 2nd month she gained around 100gms only. she still throws up, but not as before. it has reduced since before but she hasn't gained the kind of weight she should have. I had stopped giving her formula feed at night as my milk was sufficient for her. could it be because I stopped formula feeding her. I however have started giving her formula again at night. Can't notice any weight gain yet. What can I do?

P.S. I give her domstal at times and colic aid too. Regular with gripe water.