Gassy baby

My baby is just over 3 weeks old, im breastfeeding but topping him up with formula as well even though id say last night i didnt breastfeed as i didnt feel very well, he had a poo yesterday morning and ive noticed he is very gassy, his gas smells awful honestly like rotten eggs & he's not had a poop since yesterday morning, i tried some exercises with him moving his legs etc and he does pass gas but not pooping, is this a normal thing or should i be calling doctors about this, im just worried