You yourself have found out the reason for your lower back pain (LBP).... yes definately if you had LBP before then this change of vehical and seat mechanics may be a contributing factor.... also if you commute a lot on 2 wheeler ...there can be a lot of strain on the lower back muscles The immediate steps you should do now would be 1)get an Xray done AP and lateral view of your lumbar spine 2)wear a lumbosacral belt which fits you correctly when travelling long distance on bike 3)pain medications or muscle relaxants arent very helpful but a topical application of gel containing diclofinac would bring immediate relief though not for long term 4)check your serum calcium levels 5)IFT vector 90 for 20 mins would be beneficial 6)once pain subsides static back exercises targetting lower back should be started. 7)postural correction is a must Hope that answers your querry.

Change of bike might have altered the Biomechanics of your back. Probably you may require postural correction. If you have been experiencing back pain even before change of bike, then it is like a preexisting back pain that got aggravated now. Consult a physio for further assessment.