Momentary fainting and blindness?

So, I've been fainting occasionally and I'll lose sight for a moment when I fall, but after a second or so I can get back up. It's usually when I stand up too fast or raise my hands over my head. I would normally say that it would be because of low blood pressure, but my blood pressure is not low or particularly high. I'm fairly healthy, but I used to have severe asthma as a child and I normally have low oxygen levels. I think I might see my doctor, but I thought I'd ask on here first to avoid wasting money.