A story from internet:

Yesterday (when I see my doctor), a girl before me in the queue had a strange symptom. Her nose is always blooding (not really blooding but only some dried blood when waking up) during period. The doctor told her, it is very likely to be 子宫内膜异位症, the desease is very rare, she also gave other examples and said, someone always has her finger blooding during period because once during her period she had her finger cut, and after that the finger always has period. The doctor told her not to make love during period. The doctor also said making love during period makes the period blood back flow, which I sensed very Chinese-medicine- doctor-like. At last, the doctor said, the only way to cure the desease is to give a baby.