I too have the exact same thing as you. I feel the underarm numbness/pain and tightness across the left breast area where I had a mastectomy. I've read, and asked my surgeon and my oncologist about it and spoke with support group participants too. They all say it is normal, and the underarm numbness/pain will not go away soon. In fact, it will be a long time. All seem to say it was nerve endings that were cut and what will happen is that we will get used to that feeling.
I hope you are progressing well.
Ask your doctor if you can be referred to Reach to Recovery. They can instruct you in simple exerices and offer to support from a woman who has gone through this before. This is sponsored by the Amereican Cancer Society but requires approval of the doctor before they can see you.
Dear Happy4825: Pain and discomfort following surgery is very variable and depends in part on each individual woman. As tissue heals, scar tissue can form and this can be very stiff and painful. You might ask your doctor when you can start exercising as this sometimes help. His office staff may be able to provide you with some specific exercises to help with the pain.