See a doctor to get it checked out. I've had spells like this for about 10 years, then in April, it didn't pass like it usually does and I ended up collapsed and unconscious at work and got rushed to hospital. Turned out to be postural hypotension but it certainly scared me and you need to get it checked out to make sure it's nothing more serious.
When I was diagnosed, the doctor told me that as well as being careful to eat the right amount of sugar, I also need to watch my salt intake. Apparently, people with postural hypotension need 10g of salt a day rather than the recommended 6g. Now I'm not for one second telling you to eat loads of salt but just another reason why it might be worth getting checked out, in case you need to make a few subtle diet changes.