What is wrong with my horse?

Ok whenever there is a mare in heat at my barn my horse goes nuts! lShe starts snorting, trotting and arching her neck like a stallion. She will run up to their back ends and smell their area and sure enough the mares start winking. She actually has tried to mount a mare once..... What is wrong with my mare? I have been wanting to ask this question for a long time but it is a little imbarrasing.