brown spit up?

Any of your babies have had any brown throw up? My baby almost 4 mo adjusted started out with a tiny bit of watery spit up resembling coffee/coke, after feeding he threw up this brown spit up while burping. Later this super dark brown! I've taken him to two different ER's and they say he is fine. The first one didn't really do anything besides take his temp and vitals and listen and feel his tummy. Second one did take an x-ray but said he was fine! This all happened tuesday night/into the morning. Didn't happen again and both places said he looked fine. I take him to his ped today(thursday) and she says that it could be a stomach bug but he looks perfectly fine! he hasn't done it since that tuesday night when it started so fine. But tonight he's waking up frequently crying and gets back to sleep, I did notice it sounds like he's trying to catch his breath almost? maybe needs to burp, I pick him up and he throws up again, very light cream colored brown but still! Could this really be a stomach bug? he does take those iron drops every so often that are darker brown color but hasn't had any since last wednesday. Either way, he's never thrown up like this before. He's did have pretty loose bowel movements monday/tuesday almost every diaper change. No fever, no hard tummy, still a pretty happy baby or other changes besides getting some eczema breakouts around his face and body! Could it be the iron drops but why now and not before? 😢