Is she cheating on me or not?

my GF & I had a fighting regarding coming sms other cellphone that she has at her home..we are together for about 1.5 year..we love eachother and...but from the time that we made friendship she di not brought that cell phone any more, and she got another cell number..suddenly 3 weeks ago when i was on the other her cell phone i heard sms came to another cell phone..I asked her why she did not tell me she is using that number and if shthat cell phone why she does not bring with her for at least 1 year any more. I should say that she does not work and she is just studing..is she cheatting on me??after that fighting she did not answer my calls and I asked her to come to talk..but she did not accept..I told her I loved her I do not want to loose her but she left me very easy...