Splints out 7 days after septoplasty, could breath fine, 3 days later and one nostril is shut. Did my septum shift back?

It is difficult to answer your question. The most likely cause is swelling that should resolve over time and your breathing should improve. There is a small possibility that your septum could have shifted over, but you will not know that for at least a few weeks. There are some rare things that can occur that can be causing the problem, such as bleeding within the septum cavity that could cause swelling. If that is the case it is probably something that needs to be evaluated sooner rather than later.  The recommendation would be to let your surgeon know. They should be able to tell with a quick examination whether there is anything significant or its just normal swelling.  Good luck!