Vienna is correct, it could be so many things and we really can't tell you what's happening. This looks like a discussion to have with a doctor.
I would add that is you have such symptoms, fainting in the presence of a fast heart rate needs to be checked out to go to the ER to be safe.
Good luck,
Jon
My first guess is: you or whoever that is happening to was eating a huge not so healthy meal. :)
But seriously: it can be many things. If it is continuing, visit your doctor.