The disconnect that you feel is something most of us have experienced. Emotional level is totally different to your work colleagues at present - as you get through treatment and settle into the new you perhaps that connection will present again! Please don't feel sad about it, it is what it is and as well meaning as everyone at work maybe they truly are not on your emotional level and hence that appears to be your struggle at present. It will pass and you will settle! In the meantime you have this forum full of all of us who get it!
Take care