Sounds like something I had a couple of years back.
I injured my back at work lifting stuff that was a bit to heavy. First af all, a doctor is a good idea, just in case. I did learn that herniated discs are unlikely for the young because it take years to put that type of where on a disc. It is very likely that you have strained the muscles in your lower back, they are some of the weakest in your body.
The pain is caused by an inflammatory response in the area creating pressure on the spinal nerves. I, being the man's man that I am, continued to work through the pain for months. Than one day like you described, it just spazzed out and I was in pretty bad shape. I was actually bedridden for a few days. My doctor gave me anti-inflammatory medications which eased the pain. I was out of work for three months. one thing I learned the hard way is that you have to allow the back plenty of time to heal. Do not go being The World's Strongest Man just because it feels better because the pain will return instantly.
After my back got to the point where I could move again, the doctor started me on some real basic exercises. One of them was to lay on my stomach with my hands behind my head and lift my head and shoulders up. This builds strength in the lower back muscles with out exposing them to injurious strain. Another was just lying on the edge of the bed with my head and shoulders hanging off to arch my back. Something about stretching the connective tissues. Then he put me in one of those Nautilus type machines where you put your arms up against some bars or what ever and twist at the waist, like others said to strengthen the abdomen.
Most importantly he told me NO HEAVY LIFTING. Kind of tricky for a cabinetmaker, but in due time a figured out how to get through life doing as littlie lifting as possible.
If you do not want to see a doctor, give your back plenty of rest. Take an anti-inflammatory and when you feel well enough try some light strength building exercises. But, dammit Jim! I'm a jointer not a doctor!
You really should see a doctor. I've toughed my way through some broken bones and other sh!t but this back thing brought the baby out in me. Plus it seems like back injuries become chronic problems relatively often.