I do not know what is going on with your back. I am not a doctor. For a long time I had to ride really upright, because it felt uncomfortable to have my handlebars in a low position. I have been on a back/ hip strengthening regiment. It has made a huge impact on my cycling. I have now lowered my handlebars three inches, and my hips/ back are fine. I am short - 5'-6" with a 31" ground to saddle contact point underneath. I used to ride 52 cm road bikes. now I will not consider anything bigger than 44cm. (much nicer) It is possible that you could go significantly smaller. (if necessary)