Help please!!

I'm going out with my partner over a year now, she is lovely, good-looking,funny,kind, caring everything that you would want a partner to be. everything was going great until about a month ago. you see this girl has a bit of a past, I know she had slept with a number of people before we start going out but I trusted that she changed when we started going out with each other, The reason im questioning this part of our relationship is because my friend told me he heard a rumour that she was getting very close to a person in her college and that she made a new friend at the gym who has been texting her so then I started to question why she never mentioned any of this. How can I approach her with this without her thinking im accusing her of the past, its a very sensitive subject.