A lot of this depends on how long you have been dating, and how serious it is. And I think it also makes a bit of difference how old she is.
I would give her the chance to explain to you why she felt a need to see this guy. Without knowing, a logical assumption would be she wanted to see if there was still something still there with the old boyfriend. But it could be something else, so you need to find out what that is.
I don’t think it is the same thing in your case, but in my past I had a similar situation.
I had broken up with a guy I had been dating off and on for two years when I met my now husband. After my husband and I had been together for about 3 months, the old boyfriend called and said he wanted to meet for a coke somewhere so he could return a few items of mine he had, and I could return a few things of his I had. I could have boxed the stuff up and mailed it, but I decided to meet him. I didn’t ask my husband if it was ok, and I even considered not telling him at all. But that didn’t seem right. So I just told him I was going. I didn’t expect him to be bothered by it, but he was. He just couldn’t understand why I would want to see this guy. They guy treated me terribly and I was glad to be away from him.
At first I didn’t admit the real reason I wanted to see the guy. I felt it was a bad reason and was embarrassed to admit it. But when I could see it was making my husband uncomfortable, I fessed up. Once he knew, he was fine with it.
And what was the reason? Again, I hate to admit it, but it was very simple. The guy was constantly nagging me about my weight. I was about 40 lbs overweight. He told me he could never consider marrying me until I lost the weight. The constant criticism about my appearance was very hurtful to me. For two years all his insults did was make me depressed. I tried to lose weight, but eating was how I dealt with depression, so I didn’t have any success.
After I finally broke up with him, and met my husband, and fell in love, the weight came off. Being in love gave me the energy to work out, and I was really happy for the first time in my life, so I didn’t need to overeat to deal with depression. So between the weight loss and that glow you get from being in love, I knew I looked great for the first time in my life. And it was rather shallow of me, but I wanted the guy to see how good I looked, and how happy I was without him.
He was very cruel to me when we were together. He would often tell me that I better make all the changes he wanted me to make because no one else would ever want me. If I was lucky I would be with him, if not him, I would be alone. He played on my low self esteem and made it much worse. I couldn’t see all that when I was with him. But after being away from him I really blossomed.
I agreed with that old saying, living well is the best revenge. I was now living well and had my revenge. But part of me needed him to see also. The fact that I wanted to show him I was so much better off without him didn’t mean he meant anything to me. He really didn’t. I had nothing but bad feelings about him, and nothing but wonderful feelings for my husband.
So when I explained all that to my husband, he then understood why I agreed to meet him. He could never have known all that if I didn’t tell him.
So maybe there is an explanation, she just didn’t feel a need to go into on the phone. I would first let her know you need to know why she felt a need to see him, when she knew it would bother you. From there I think you will have a much better idea where your relationship is headed.