What is the reason for the big bag after cupping?

Cupping

is the suction of negative pressure generated by fire, suction, steam, etc., to suction subcutaneous tissue into the cup, in order to play the purpose of activating meridians, circulation of qi and blood circulation, dispelling wind and cold. However, due to negative pressure adsorption of the skin surface, can lead to local capillary rupture bleeding, thus forming subcutaneous hematoma. At the same time, the skin of the hematoma site often local bulge, touch there is a feeling of fluctuation. In addition, if cupping is carried out by burning fire, the temperature may be too high, resulting in local blisters.