Grey lines on front teeth like drawn from a pencil.?

I m a mom of 7 month old baby. I do breastfeeding since the birth of my baby.I ve got grey lines on my front teeth.it s been two months since I got it. It s more visible now.can t scrach it off.is it due to iron deficiency? I took iron supplements and calcium, omega 3 until my baby become 4 months old. Please reply me.I m so disappointed since it my teeth.:(