Why is my friend acting like this?

So my ‘so called’ friend is normal when its just her and I or if we are with our other friends,but she goes all b*tchy when a guy friend a like and have chance of dating comes near(we are actually together now) This same guy would always hang out with us, and she was fine, but as soon as we started to talk more(may I add..I did not ignore,ditch, or avoid her to talk to him) she went all blahh!!! I don’t know what the prob is. She always does that though..We think she may have a crush on me(trust me its possible!) or she is jealous of me. But why? She has always been my number one friend even through the roughest of times and she just gets mad at me. Im not hiding anything from her and its not like she cant get out and get a boyfriend(or girlfriend) When I ask her wats going on, she cant seem to answer me or tries to blow me. See I always reassure her that I care about her like a good friend should. I just feel like giving up. Im sorry its a lot to read, but I wouldnt be writing all this if I truely didnt care. I hope you can help shed a little light on whats going on, I would really apprieciate it!