Lower back pain/plantar fasciitis?

I have been having severe back pain on and off for a while now. It is in my lower back (worse and more frequent on left side). It usually occurs when I have been standing for long periods of time and is accompanied by aching pain in my feet. The pain gradually gets worse and eventually becomes unbearable. If I sit down or lay down and then try to get back up, I get a sharp, stabbing (excruciating) pain while trying to get up. The pain gets much worse after I am able to stand back up and often I am unable to straighten my back completely for a couple of minutes. The pain is worse when standing on left leg and is somewhat alleviated when all weight is on my right side and I lift my left leg slightly. Laying down/sitting does little to help and concentrated pressure makes it feel slightly better. I was told it is plantar fasciitis because I have flat feet. Is this true?? It is EXTREMELY painful.