Who experienced passing out or nearly passing out ?

Va

I've been counting calories for about a month now. And been having heavy bleeding due to pill and I did some exercising this morning for the first time. after an hour I was playing with my baby who was in a bouncer and felt something weird in my chest, like my heart kept skipping a beat, got scared and got up and everything start going dark and I thought I'll pass out, but everything went away very quickly... what could it be?