Fainting while lifting weights

[BirdieLongSocks]

I have started to go to the gym on a regular basis. I prefer repetions over weight so I usually do about 15x3.

Here is the problem, I faint. It goes black and after some seconds I am in the game again. I get enought time to A, grab something B, sit down. I dont faint out of exhaustion, could it be blood presure relate? lack of vitamins or minerals in my diet?

Thanks in advance