Two months ago, someone is assisting (with another person) to get someone lifted off a chair. They do not get severe pains. As time goes by, even without the person around for two weeks, a couple weeks later there is no pulling of any muscles, tendons or joints.

Then a week ago today they start to receive sharp pains on one shoulder blade, it travels to the other... almost completely goes away on Tuesday and comes back Wednesday and then travels to their head. Pain is also evident in the left arm at times.

Would you think this whole situation would have begun in August, rather than a week ago if it were solely straining and pulling something?

Thus, be more psychological related to an issue (that happens to be related to the assisting of another person) that is taking place at the same time?

Stress can cause pain. (so can lots of other things of course. It would be important to rule out other health conditions that cause similar types of pain first e.g. heart condition)

Stress and its effect on physical pain
from PsychCentral

Stress: How It Affects the Mind and Causes Pain
By Dr. Mark Wiley, The Healthy Back Institute
A Cyclic Back Pain Example...