Blood in sick

Ive not been sick for over 2 weeks now (which is great!) but this morning i woke up first thing this morning feeling really nauseous, i was then sick and it was mostly water (since i havent eaten anything) but will thick dark blood in it?!
I have been given iron tablets due to low iron and have only been taking them for 2 days, could this be the cause??
I am 14 weeks