I agree with Jay’s advice regarding getting rid of the sand substrate and switching to paper. The best case scenario is that it just got a mouthful of sand. Is it possible it still has some skin attached to its snout that it was trying to rub off when it last shed? It kind of looks like that in your photograph.
If you are able to restrain the python safely and keep it from thrashing about and injuring itself (it is nice if you have someone else to help with the holding), then you could take a q-tip and swab gently from front to back on that right side of its mouth. What comes out? Hopefully, just sand and not necrotic tissue.
You could also open its mouth and take a further look inside. It is helpful if you have looked at healthy, normal mouths before looking at one you have questions about. But you really don’t have to be an expert to know something might be wrong if you see something like blood, or masses of cheesy-looking gunk, for instance. If you want to do this, make sure you use a small, perfectly smooth, cylindrical object which you can hold gently across its mouth. Lacking that, you could use a small, rubber kitchen spatula.
-Joan