3 weeks old baby extremely fussy and crying alot

Hey everyone my lo is 3 weeks old now and he started showing fussiness and irritability . He is not pooping as usual. He only pooped once in last two days. I am breastfeeding him and use fm only once in a day. It looks like he push really hard while pooping and turns red and starts crying. May be he is constipated. I also tried burping him gripe water and tummy time with massaging his tummy to relax but nothing is helping. I feel so bad to see him so uncomfortable and in pain where i cant help him. Please give me some advices what to do? Should i change my diet? I am purely vegeterian and drinks alot of milk (2-3) cups/day.