Stress about baby's milestones

Anyone else feel a little stressed about your baby's milestones? I have a friend who gave birth about a few weeks after I did. Her baby could roll over at 3 months, can now sit up unsupported, and has teeth! As for my little girl, she has none of those. She can somewhat sit up by herself unsupported. My mother told me that my child's lag in development could be due to my lack of iron or vitamins in my system, so when I breastfeed, the baby isn't getting enough nutrients to develop well. Could this be true? She's almost 7 months and is fine in birthweight.