Does this sound like a seizure?

This has been happening off and on for the last two years, often multiple times per day. It usually happens upon standing up but will sometimes occur while running or very rarely when lying down. Every muscle in my body goes completely limp for a second or two and immediately after my toes, feet and sometimes legs will violently tense up for a few seconds aswell. I can feel it coming on right before it happens so if I'm with others I can fight it and remain standing but I must have the stupidest look on my face as my facial muscels briefly lose all tension. When I'm alone I prefer to laydown and let it do it's thing rather than fight it. I never lose consciousness and am completely aware, the process lasts no more then five seconds, and I've become so accustomed to it that it's never really bothered me. My friend suggested they could be small seizures so I decided I might as well check it out. Any suggestions as to what's happening are appreciated.