What Causes Numbness In The Body?

I have in the last 2 weeks become numb from the breast down and its spreading up to arms, front and back of body, its like i have had a giant novacain shot i can tell im scratching but cant feel it. Im very sedative with little to no exercise, i have a bad back and have had surgery on both knees what could cause this numbness so suddenly? I am 48 yrs old and about 60 to 70 lbs overweight