Hello,
I am new here and I experience the exact same thing. This has been going on for around ten years now. I have been to many doctors and a couple chiropractors. I have never had an answer as to why I go thru this pain. One time I had this happen to me when I was at the chiro. He told me my lower back had a severe muscle spasm. Since then I have been giving a high dose of muscle relaxers and it helps a lot but it does not "cure" my problem. I just recently went to the doctors again and this time a demanded to know why I go thru this pain. Whenever my lower back has a "muscle spasm",(a severe sharp pain like being stabbed) at the same time my legs give out on me. I have to do the same thing and re-adjust myself on the floor to relieve the pain. I am scheduled for an MRI now so hopefully I can find an answer out soon.