Aw, iron supplements talaga ang usual culprit dito, pero since it's been a week pa rin kahit na-switch na ang meds, kailangan mo talaga i-follow up with your OB ulit, sometimes it takes a few days pa para ma-adjust ang system, or baka may iba pang causing it na kailangan i-check. One of my mum friends had the same issue at nag-adjust din ng timing ng vitamins (taking them with food, or separate times) which helped her, pero in your case mas better na i-consult ulit since persistent pa rin siya.