Never consider that you are paranoid! Never!
As we make our way through this we have an heightened awareness of our bodies and my GP always says come in, don't wonder!
Sorry that the Physio put you through laying on your tummy. My mastectomy site still gives me curry and I can point directly to where the drains were; will it ever settle? Damn well hope so!
But back to you, no, no, no you are entitled to get to the bottom of why and if it is muscular so be it! At least you can be satisfied and find a way to work around it without being compromised on a table upsetting the mastectomy site.
But until the scan who knows but here's hoping all is well!
Take care and sending you a virtual hug