I Would Use And Dump Her!

I am a guy of 28 years who just finished youth service in June and newly got a job. Around July, I met a girl that I am currently dating.
Prior to this day,  I have not really found someone I love more than this girl. She loves me too, though  she is a student and works as well but I have noticed that there is this kind of way she answers my call sometimes , so much so that I tend to suspect that she is with some other guy.
She is no longer cheerful on phone and would just wish I dropped the Call sooner or she would say “ okay bye! we will talk later” and stuffs like that.
Where she lives in school has no network, so she goes somewhere outside her house whenever she feels she wants to talk with me.
 I used this knowledge to monitor her whereabouts because when her line goes through, then I know she ain’t at home. For two days now her number has been going and from the background, I deduced that she could only be in a room and not outside her house. When I asked her about my observations, she fumed and hung up on me.

Afterwards, she would call back and apologize, telling me that she was in her neighbours room.
I love her and just wanna concentrate on her alone.  With what’s going on, do I ignore????
Some guys think she moves around. Though I haven’t caught her red handed. But do I concentrate on her cause I feel like using her and dumping her RIGHT AWAY if I get to know that she wants to play games.

What do you advice? I need urgent reply please. thanks