It could be the Iron tablets but as you're pregnant it could be hormones, change in diet or even just a simple colon infection.
Go to your doctor as worst case scenario it could be cancer, chrons or proctitis.
It's better to be safe than sorry, especially in the early stages of noticing the change, my dad left it for years and then was told he had proctitis.
Like I said though, could be nothing but if the discharge turns to blood or you get abdominable pains get an emergency appointment as you will be admitted to hospital for certain tests as that is a sign of infection or cancer.
