It is the location where referred pain is felt. Endo that grows on the diaphragm for example causes the pain from there to be felt in the under shoulder blade zone. Usually the right shoulder too.
Same with pain from trapped gas after you have had a lap op, the gas rises up the diaphragm and gets trapped there but you feel the pain in the shoulders.
It is very rare indeed, compared to the usual locations of endo, but an MRI scan should pick up endo on the diaphragm. You can then either supress the endo by hormones like BC pills, or you may want to have it surgically removed.
A rarer still but actually more hazardous form of endo is when endo is found in the lung area itself. This can lead to collapsed lung with a period and because the pain of that endo is also felt in the shoulders too, with or without lung collapse, this is definitely something needing checking on. The terms to look up in the search box on the green bar are
catamenial pneumothorax (lung endo )
There have been a number of previous discussions about these forms of what is termed thoracic endo.
Speak to your GP about the symptoms and ask for a referal to an endo centre
where they will scan you for signs of both diaphragm and lung endo.
The fact that this is happening each period and is in the same spot - would indicate that it is probably an endo thing.
One option you can try for yourself is stopping your periods and then see if the shoulder pain stops happening while your periods are stopped. e.g. take the bc pill back to back for 3 months.
Or if you think it may be muscular, and perhaps the awkward stand you take if you insert a tampon for example- can be tested by switching to just using sanitary towels for one period. It can be that you do contort your body in a strange way to insert a tampon and that might aggravate a shoulder muscle. Or switch to inserting with the other hand instead. Not as straightforward as it may sound though.
I broke my wrist in my 20s (slipped on an icy pavement) and had to use my left hand for tampons and my god it was not easy at all to figure that one out. so towels is probably the better one to try LOL.
at any rate do not leave it unchecked out. There are things you can be doing to diagnose and hopefully solve the problem. Best of luck.