Gray Poop!? What the.....

Courtney
Hello ladies! 
Has anyone ever opened a diaper to find GRAY poop!? I was shocked when I saw my LO's today. 
I am in the process of weaning off of pumping, so she's had a little bit more formula than BM (she's had formula since we were in the hospital). But other than that nothing has changed diet wise. 
She had a cement colored gray poop around 9am, then 2 normal yellow ones, then around 6pm she had another gray one. What gives!?
Anyone have any experience? I'm getting pretty worried. TIA!