Why would my gf secretly call her ex husband from a private number?

My girlfriend called me accidentally the other night with her phone set to private, she lied and said she mistakenly set it to that, but I didn't buy it and eventually she said she was thinking to call her ex husband whom she has been divorced for over a year and a half from. She said it was because I seemed a little distant, (even though we talk several times a day). The last time she admitted to talking to him was in December which is only a few months ago, and assures me she hasn't since. The "private number" issue was only about 3 weeks ago. She actually gave me her FB password and told me to look on there to confirm, I humored her and did so. I guess she forgot to delete her sent box because there was a message to him about a phone number and was asking him to "look again please, please". I didn't tell her I saw this. I could also tell that several of the threads had been deleted on that set of messages. Funny bc 2 days later she had sent out messages to her friends about her new phone #. We were on a break and started talking again about a week after the # change, and she lied at first as to why she changed it, then lied again, then told me that she called her ex while we were on a break, then changed her # so he wouldn't have it. She assures me that he doesn't have it now. The thing I noticed about her messages when I looked at them is they are very consistent but there is an 18 day gap between messages in January and I'm wondering if she deleted any incriminating stuff? Also if she is still talking to her ex, why make her # private before calling him?? I don't understand the logic behind that