My back has a spot, painful,swollen,itchy,even not pressing by fingers,it is still painful.suddenly happened at the office today.

Boil/abscess: This sounds like a boil or abscess, however we cannot be absolutely certain without seeing it. Small boils and abscess usually can drain with repeated warm compress for 10-15min Five times a day, but since it is on your back it may be difficult if you don't have a willing family member/friend to help you. A long bath may help. If it is large, >1cm, you should see your doctor for drainage