4 weeks old and stomach issues

My son just turned 4 weeks and I’ve notice the pass two days his stomach makes a gargling sound, he is passing a lot gas/burps and when he tries to poop he strains and nothing happens but just a lil Mark in his diaper . I’ve been able to help him by using a thermometer ( like you would to check temp ) and he will eventually go like that with time . I’m just curious on what food could cause him to have a upset stomach ? What can I do to help him?. I feel so bad for my little man . When nursing he actually burst out crying and straining. I haven’t gotten much sleep due to him crying and wanting to lay on his stomach across my chest.

* btw his poop is not hard .. it’s the normal poppy seed looking poop when it comes out. Makes me wonder even more why he struggles *