Why do I have shoulder pain after doing push-ups?

Since I began doing pushups, I've noticed a sometimes-dull sometimes-sharp ache in my right shoulder when I’m going about my day. What could this be from?
I recently started working out more frequently, doing 3-5 hour long yoga sessions every week and trying to build arm strength by doing 30 push ups (with my knees down)…

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Why do I have shoulder pain after doing push-ups?

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