I've had the same problem, repeatedly surfacing ...
I think it's because I overdid one of the stretching exercises which seems to be borrowed from Yoga...
And overstretched lower back muscles.
Question: Any suggestions on how to reduce the frequency of recurrences of lower back pain?? I seem to get it about every 5 months. It's not a slipped disk or anything serious, I believe it's either pinched nerves, muscle spasms, or both, and I'm only 31 years old, but ever since about 3 years ago I've been experiencing this. I've been training 3 times a week since the first week of October, and I have found that the warm up exercises at the beginning of classes do help. I try to do some of those exercises at home in the morning too. But, I just went through my first aikido seminar last weekend and woke up this morning with sharp lower back pain and my movements are limited, and since the seminar was mostly weapons training(two-sword technique, Mike Lasky sensei), we didn't do much ukemi, so I can't see the seminar being the problem. Anyone else share the same problem?? What exercises help specifically?? Thanks for any suggestions.