Generic form of cream and it works for me on the lower back pain. Like you- The main cure is to stretch out over the bike so a new bike is on the cards for you. Longer top tube and lower the bars by another 2".
Newcomers to riding and then back pain will not take the step of getting longer and lower but it works for too many of us to be ignored. When I first started on road bikes after MTB's- I got a frame that would give me a position similar to the MTB on the flats. Then raised the bars to be level with the saddle and back pain came in at about 20 miles. Not serious but it was there. Next bike and the shop built it up. Longer top tube and bars 4" below the saddle. Test ride proved they were right and 3 years later they are still right.
Attachments are of the two bikes so the difference can be seen.
B2..jpg B3..jpg
But onto that back- I also do some stretching exercises to keep it mobile and I make certain that when some heavy lifting is about to take place- I am not there.