Hi I Am A 60 Yr Old Female. Obese And

Hi
I am a 60 yr old female. Obese and not very fit for past three years or so.

I was doing some very light arm exercises with 2KG weight two days ago and felt a sharp pain in my left upper arm. The pain then went and I had no further problems.
This morning, though, I have woken with severe pan in my upper arm and shoulder. Not so much at rest but when I try to internally rotate my arm, especially if it is abducted. As a result I can't reach my hand behind my hip, not get my hand onto the waistband of my pants on the left side.
What can I do? Can I treat this at home? I cannot take anti inflammatories due to a long gastric history. i am taking Paracetamol and using Zen spray (a menthol concoction)
Thanks

Annette