or from friction, which create a tear between the epidermis—the upper layer of the skin—and the layers beneath. When this happens, the surface of the skin remains intact, but is pushed outwards as serum
seeps into the newly created space between the layers.
Short periods of intense rubbing can cause a blister, but any rubbing of the skin at all can cause a blister if it is continued for long enough. Blisters are most common
and feet, as these extremities are susceptible while walking, running, or performing repetitive motions. Blisters form more easily on moist skin than on dry or soaked skin, and are more common in warm conditions.
Sometimes, the skin can blister when it comes into contact with a cosmetic, detergent, solvent or other chemical; this is known as contact dermatitis. Blisters can also develop as a result of an allergic reaction to an insect bite or sting.'
http://en.wikipedia.org/wiki/Blister
It would be best to consult a doctor if it does not resolve on its own in a few days.