Help! I'm having weird fantasies?

Basically.. i have a beautiful girlfriend and i love her to death, we both love each other so much all that good stuff. However, as of recently, i've been having this weird obsession with her best friend and i'm having dreams about her best friend texting me and i'm like waiting at the phone just for her best friend to text me (it's not something she normally does, this is a completely obscure thing to do) i love my girlfriend but i don't know why i'm feeling like this towards her best friend! I don't know what is going on inside my head, can anyone lend any incite?