stool changing colours and smelling bad

yesterday and the day before, my poo was black (I took an iron tablet two days before so it could just be that) today, it was yellow and had an awful smell. (this is really gross, sorry) after 4 hours I could still smell it on me so I had a shower. after my shower, I can still smell it and when I wipe my butt, theres yellow marks on it no matter how many times I wipe, but I can’t see anything there.

can this mean something? I’m really worried. I’ve had on going stomach issues for the past few months as well, constipation, diarrhoea and nausea.

please help.