Is she cheating on me?

Please help, I have one problem...I am in a relationship 6 months, but in last few weeks my girl, started acting strange, I am wondering if she maybe have someone else, is she cheating on me? another guy in her life maybe? I aksed her, but she said that I am talking silly things..and she said , that this is just her bad period in life...HELP ME PLEASE! what is going on?
here is a chart on the link below...THANK YOU VERY MUCH

http://postimg.org/image/mimevb2mj/