I answered in another thread, but just in case you didn't see it -
I had the same problem a few years back with my sciatic nerve. It got to the point where I could hardly walk (at 19) and then when I tried to bend over to pick up a sock or something like that I felt this snap in my left leg and a red-hot flash of pain went down my leg. I thought I was bleeding, but I wasn't - went to the doc and was diagnosed with a herniated disc in my back and sent home on bedrest for two weeks. I was fine shortly after that, but a few weeks later while shaving my legs, I noticed that I no longer had any feeling in the back of my left leg. Quite an odd feeling while shaving and then you don't feel the razor anymore! I ended up damaging some nerves in my leg from when I bent over and felt that snap. Still don't have all the feeling on the calve of my left leg and the last two toes on my left foot, but it has gotten considerably better over the past three- four years. Nerves grow back, they just take a long time.
You probably overexerted yourself in some regard, or maybe its just that your body is developing more muscle in the back and its placing some kind of pressure on the nerves. Sometimes I don't have full feeling in the one area of my leg that is more sensitive because of over-doing it, even though my back doesn't hurt. Though I am by no means a doctor, if it gets worse or progresses, I'd go in immediately. Otherwise, maybe lighten up on the weights you do for your back? Even though it may not seem like a lot, your back is sensitive.