What Causes Knot And Pain Under Shoulder?

I.m having pain under my shoulder in my back also a knot is feft in that muscle for about 3 weeks nowI ve been walking up to 2 miles a day swinging my arms is it possible it s a pulled muscle under my shoulder blades, I use my arms held up quite abit because I m a hairdresser and I m 63 years old, nothingI try seems to help for long but I can feel the knot there YYYY@YYYY