21 F Constant Dizziness

I'll start this off saying I'm female, 21, 5'6" and about 273 lbs, I don't smoke and I have asthma.

For the past week or two i've been dizzy on and off, before it was only happening sometimes after I slept. Which led me to believe it was dehydration, but now it has moved on to be randomly throughout the day and i can usually sleep it off, but it comes back. It's less of how people describe vertigo and more of like a boat rocking sensation. Or like im constantly falling. My friends say it sounds like im missing vitamins, but im thinking it's more than that. I'm hoping someone will be able to help me.