I Am 16 Wks Pregnant And Up Till Now Have

I am 16 wks pregnant and up till now have been having healthy bowl movements. I ate salmon and snap peas for lunch and soon after I have a very black water stool. I don't take iron pills. The only pills I take are a prenatal, woman's 1 a day, and omega... which have never set me off. I eat rather healthy and jog 1-2 miles 3-4 times a week. I am only really concerned of the color of the stool...