Prenatal vitamins and 'stomache upsets'?

1 day to go until im in the 3rd tri, but i think I will pop over here now if thats ok with you girls?
I was taking prenatal vitamins 5 months before pregnancy and all the way through- and my toilet habits became waaaaay regular.
I told my midwife who suggested I stayed off the pills for a week or so, I did exactly that but then I noticed that my skin was breaking out. Being off the pills meant that I was going to the loo for a number 2, just once a day, and didnt feel the urge to go anymore than that! Well I went back on the tablets last night and now I feel I need to poo all the time, plus, without sounding too disgusting, my poo is really really dark compared to how it is when Im not taking them.
Anyway, what I was wondering is if anybody else experienced this weird need to poo when they were taking prenatals?