Regarding 20 days baby spitting milk.

My baby is 20 days old and I'm doing breastfeeding from day one. Baby weight was 3.1 when born in c section and it got reduced day by day and again after last week it was 2.96 and today it is 3.1 kg. My concern is he don't sleep at night and morning he sleep but from last 2 days he didn't sleep and was crying. I gave grip water but no use. Doc advised to give colic aid but no use so went to doctor he gave domstal, lanzol tablet ,tummy softener and continue colic aid . I gave tummy softener and colic aid but still he didn't go to poo. He is frequently doing pee but still crying and also from first day even after burping him he spit out the milk and do vomiting often in a day. I'm very much worried. He looks thin. Please do advise if any problem. Also I'm suffering from constipation will that be an reason for his cry as he drink breast milk. Please do advise