I Have Recently Been Experiencing Moments Of Feeling I'm Going

I have recently been experiencing moments of feeling I'm going to faint. No dizzy, just at the point of falling over.. getting dark and not in the present.I seem to go dark. I'm on med. for A-Fib, blood pressure,warfin. what could this be from or why am I feeling this way. It comes on suddenly, either sitting or standing or walking. I have an appt. with my cardiologist in a few weeks. I'm concerned. thank you