Black poo?!??

So my son is 2 and I started giving him iron yesterday and he’s had more today, we mix it with juice (per our drs instructions). We’ve given him iron before so I know what to expect or so I thought. Earlier today he had a hard poo it was darker in color and had some black and mucous red stuff. I figure it’s just from the iron I’ll keep an eye on it. But he just had a super black poo like it was all black like a mushy charcoal paste. I’m worried and it’s the weekend so I can’t call the dr. I’m thinking of stopping the iron but does anybody know if this is more then just the iron?? Should I be worried???