Sometimes I Will Be Walking And All Of A Sudden

Sometimes I will be walking and all of a sudden I feel like the blood is draining out of my legs and I have to quickly find a seat to sit down. I will wait a few minutes and then it goes away. Is this a circulation or nerve problem. It scares me when it happens. Doesn't happen very often but when it does, I panic.
I do not want to pay a $15.00 monthly fee. Will pay only one $15.00 or my answer. How do I do that??