Girl friend problems

Ok, here's my story. Dated this girl for about 3 months and fell in love with her. Actually, I thought she was even crazier about me than I was about her! Out of the blue, she says she's not into it anymore and boom, she's gone. About a month or two after that she starts pursuing me again. I blow her off for about a month and we get together. We've been hanging out for about 3 months again and I've noticed she's starting to distance herself. We get together about once a week and have a blast, plus the sex is awesome! Any advice?