What Causes Dizziness, Spotting And Fainting While Doing Physical Activities?

I get dizzy and see spotted during different activities, almost to the point of passing out. I am a decently fit and active person, but I have had this problem while bike riding, jogging, just doing armband exercises and even sexual activity. It is happening moreoften now and has me scared to participate in any activites to avoid this feeling. I have to say that when this occurs I am not pushing my body to the limit, sometimes I'm lightly participating and still get this feeling. The doctor said I have lower blood pressure and I'm not sure if this may be part of the problem. I have tried to drink more water, I eat on a regular schedule, and so I'm wondering what the problem scould be.