Well there are a number of things that make your back feel like it has given out, but I thought this site might be a good one for you, to explain it better than I can.
Sorry your having this problem, and it sounds like it's time to get it checked out by a DR. I went like this for several years before it became a constant source of pain.
Moderator Chronic Pain Forum
"Snowflakes are one of nature's most fragile things, but just look at what they can do when they stick together"