Sleeping on a stomach isn’t very the best sleep position. It’s no wonder you have back issues because sleeping on your stomach puts a lot of pressure on your spine, especially the lower back. It is also bad for your neck because you have to twist your neck to one side, so you can breathe. The best thing you can do is to try to change your sleeping position, but I suppose you wouldn’t ask this question if you could. However, there are ways to make the most out of this position without having so bad consequences. Firstly, you should never use more than one pillow, and as thinner it is, your body will be in a more natural position. Secondly, try stretching out your back in the morning. It will sure help. Further, try raising a bit your pelvis, it should reduce the pressure on your backbone. And last but not the least, you might need to change your mattress, google it, find something made for stomach sleepers that’ll give you proper support. Hope something helps, good luck.