26 weeks and 1 day pregnant, A concern? Doctor said not to worry?

This is my third pregnancy I am at high risk do to a complete previa. For those that don't know what that is (Complete previa where the cervical os, the mouth of the uterus, is completely covered. Partial previa where just a portion of the cervix is covered by the placenta. And the marginal previa that extends just to the edge of the cervix) As that maybe everything is going find. My baby is growing the way it should and everything. But, last week I had experience something I never have before while being pregnant or not pregnant. I was taking a shower, I had gotten lightheaded and dizzy before trying to move out the shower, As I went to grab my towel my back hit the wall. As I stayed seated for a while, I finally thought I might be ble to stand up and when I did I was shaking went to lay down and after all that I had gotten very tried. I called my doctor and he said no biggie it happens. I don't think so. Has anyone ever experience this? Should I be more concern? Do you think I should get another opinion? Also, if you had this happen was there something wrong? And If so what was it?