Im straight but have a crush on a lesbian friend?

okay im straight and i know i like men. i have a boyfriend at the moment but we have been having some problems. a good friend of mine is a girly lesbian, and ive started to like her emotionally and physically. We connect so easy and some how im attracted to her. Shes the only girl im attracted too and i cant think of liking another girl in the same way.I dont know whats going on with me. Does anyone out there no what could be going on? :/ any advice would be really great. Is what im thinking wrong??