Hello Mike and Welcome to Healing Well and the A & P forum.
Your pains are real even if anxiety is the cause so don't doubt yourself on that score. :)
There are likely to be other possible causes for the underarm pain, so ask your doctor about your symptoms. I can think of several causes but I am not a professional so please rule out the medical possiblities with your physician soon.
I agree with the Claritin possibly being your culprit. I have heard of people who experienced rapid heart rate, dizziness, dry mouth, shaking, sweating, anxiety, out of body feeling, blurred vision.
Again a warm welcome.