What do u think i should do?? (please read!)?

ok well my bestfriend broke up with his gf two weeks ago, and now she says she likes me,heres the whole story, he broke up with her like 2 days after her birthday and we were all goin to the movies but sice they broke up he wasint goin but i was still gana go cuz we hav been friends for a while and in the middle of the movie out of nowhere she kisses me, at first i was to stunned to do anything, but then i kissed her back i had told her way earlyer that i liked her more then a friend and she said the same back but at the time she was goin out with my bestfriend so i wasin gana try anything, but then they broke up and this happened. nd i havint told my bestfriend wat happened yet nd me and her hav been sneking around together nd im startin to feela lil guilty so im not sure wat to do