Weight is not a major risk factor for lower back pain. This may come as a great surprise, but the incidence of lower back pain in the obese population is virtually the same as in the general population. The slightly greater rate of lower back pain seen with obesity is explained by comorbidity, the combination of a number of different illnesses or symptom complexes at one time. If obesity were a direct cause or precursor to lower back pain, we would expect to find a much higher incidence of lower back pain in that population, and we don't find that at all.
Structurally, your spine is designed to carry and support many times your body weight. Of course there must be limits, and I'm not suggesting you push yourself to find out what those limits are, but the extra body weight most people carry never approaches an amount of weight your spine could not tolerate.