You could indeed be having pain from the chemo, even this long afterward. you should talk to both your oncologist and your surgeon about the pain. However, there are many other things it could be - dr's office is a good place to start to find out why the pain. You say your arms are not swelling, but you still could be developing lymph edema, especially if you have increased the muscular activity of your arms without making sure to take time to elevate them and let them drain. Most sources say women with lymph edema should keep there arms elevated above shoulder level for at least 45 min twice a day - I don't know too many women who have time to do that. Personally, I exercise with a rebounder (mini-trampoline), and I sleep with my affected arm elevated (entire arm needs to be above level of heart, with fingers highest.) You might benefit from physical therapy by someone who specializes in post-reconstructive surgery therapeutics. You would probably need a dr's referral for that.
Find ways to modify the strenuous tasks you must do because you have no help. Instead of carrying one really heavy grocery bag, split it up into two lighter bags, and carry them one at a time. Avoid plastic bags - the "handles" can really cut off circulation to the hands and fingers, and put tremendous strain on the wrists, elbows, and arms, especially if they are loaded heavily. Cloth bags are better, paper bags are excellent because they force you to pick them up by the bottom and use better body mechanics.
You might benefit all around by finding ways to reduce stress to your upper body. As a fitness instructor, you most likely are using your shoulder, arm and hand muscles a lot. You may be forcing more circulation and lymph fluids into your arms than your altered drainage system can handle. I knew I was getting lymph edema before it became physically visible because my arm and hand was starting to feel a lot different than normal, and it was starting to hurt, especially if I used that hand a lot. With increased exercise of your arms, you might benefit by wearing compression sleeves while you are exercising. They make some pretty light and much more comfortable sleeves these days than they used to - you just may have to do some online research to find what is right for you. Also, find a way to frequently raise and gently - emphasis on GENTLY - shake your arms and fingers, and do GENTLE stretching of your shoulders and upper back muscles. This will help return circulation and lymph fluids to the chest and minimize congestion in the arms/hands.
Most importantly, get it checked out by the doc/s, cause as I said earlier, there are things this could be other than recovery from chemo or lymph problem. You may even have sustained an injury as you returned to pre-surgery routines.
Good luck.