Should I tell her how i feel?

So I'm a girl and I've had crushes on girls before but i've never really came out to them and told then about my feelings. At my job theres this girl who just drives me crazy like my feelings are gettin stronger. The girl recently started this "fling" with this dude at work who is so immature and stupid. She tells me how this relationship is nothing serious and what not but it just kills me everytime she tells me "oh he so sweet ..he said he misses me" and i just dont know what to reply bc i know he doesnt mean it. well heres where i become confused. the girl and i have facebooks and everytime i log on she IMs me saying "oh baby im here at home why dont u come over and play?" and i just "LOL" back cuz idk if she means it. this one night i was drunk as sht! and stupid me IMs her telling her "oh ill give u the best head ever... i love u." etc and the funny thing is she goes along with it. so i called her that night and she was like "im just messing wit u" and i was "BLOWN". this girl always flirts wit me. at work hugs me from the back and i tell her "please stop harrasing me" and she says"u know u like it"..she also dedicated avril lavignes song "girlfriend" to me. i dont know what to do because shes wit this dude and its so annoyin wehn she tells me "oh yeah we ****** last night". i dont know..do yall think that shes just playing wit me? like shes kidding or does she have some feelings for me deep inside? should i tell her my feelings even though shes with a dude? if i tell her idk how she would react and i dont want her to stop talking to me..idk im so confused! help