try some stretches, sounds like a muscle ache / over work / pull
I usualy get it towards a hard sesh in the gym, bad ache in lower back, lie flat on back with your arms outstretched as if you were making a cross with your body, draw your legs up then over to the left, repeat with legs over to the right. Or, sit on a chair, lift heal of each leg onto opposite knee, chin up then slowly bend forward.
Best thing to do is go to the doctors ASAP, i did my upper back in at work 2 months ago and i had ripped the muscles either side. He will give you some strong anti inflammatory's which give you the shits and make you feel ******* bad if you dont take them just after a meal!
It's most likely that, but wait till tomorrow and see. mine took ******* ages to heal (4 weeks) because i was working continually. Just rest and give it a few weeks of doing no heavy lifting/exercise that will use your back as it won't heal. Even if it feels fine be careful.