Suggest Treatment For Severe Dizziness And Dehydration

At my Doctor s recommendation I was taking Iron tablets and vitamin C for Iron deficiency and Calcium tablets with Vitamin D3 for Osteopenia, when, after about two months I started to get very dizzy when changing position from laying on bed to sitting. Not so much when changing position from sitting to standing. For the last five days i stopped taking any pills and I started to drink one liter of milk per day, but there is no improvement yet. I understood that the dizziness could be caused by dehydration, postural dehydration, and I started to drink more water and put more sea salt in my food. What else I can do? Thank You!