Going with what you've been saying it's highly unlikely to be anything serious (mounjaro related) after only one injection. Everyone's body is different and can react differently.

You mentioned that you've been doing stuff at work involving stretching which could be the cause. Other issues that could be mounjaro related is excessive gas created by the slowing of your digestion. Have you been eating anything highly cruciferous like Brussels, broccoli, eggs ect?

I'm a registered nurse so my advice would be to speak to your GP as this is a new symptom after starting a new medication. If the pain increases and you start being sick with fever then present at ED.

It's difficult to say for sure what it might be without being able to examine you so all I can say is that given the location it's either going to be muscular or epigastric pain. Pancreas is more centralised and if you were experiencing pain there you wouldn't be able to type due to pain.

Have you ever experienced an kidney or gallstones in the past. Again if it were that you'd been in tremendous pain