i trusted her

i have a girl frend that i trusted so much ,i love her i know she also do love me ,she has been faithfull to me untill sh whent to stay with her aunt who had a son,i noticed that her character chanegd a little ,i was suspecting her and her cusin cos of the way they behave. asked her she was angry and was like how can i say such a thing,now one day o sour one love text in her phone , i asked her she was pisd off she thouth i will not see the text as it was hidin in.now i asked her she said that its one guy that has been disturbing her and even whent as far as calling the gys name for me.now i was able to cram the number i called the number and it was her cusin.now with the way they have been acting  and the way she reacted when i sour that text i know somthing is wrong .i told her i will forgive her but that i need to know the truth but will abandon her if she dont tell me the truth.she said that it was one day she wasnt feeling fine every body has gone to church and the cusin is to take care of her he after buying some pils for her now did it ,but that she didnt know that its after he hard done it that she came to know,that she is been trying to tell me but promised the cus naver tell anybody,now girls is it possible for a guy to use you withouth you knowing,two she is the type that is difficult to penetrat how come the cus penitrated yet she didnt know is it possible. still love her but am hunted by this ,what should i do