Am I wrong to be angry and upset about this?

My fiance and I have been together for just about 2 and 1/2 years. We have not yet set a date but are thinking something small in early fall. Anyway ... Within the past few months his "high school sweetheart" somehow found his email address and contacted him. Well they have been keeping in touch ever since. A few phone calls and a lot of texts and emails. He and I had our first child together 15 weeks ago, plus I have a 5yr old son from a previous relationship. My 5yr old calls him Daddy as he is the only "father" he has ever known. But to get back on the subject ... He sent her via email today pics of our son and a copy of his birth announcement. Her response was "he is so handsome ... I bet he will be hot like you were!" Also she asked who Colby was as he is of course listed as the brother in the birth announcement. My son has my fiance's last name (long story) This says to me that he has told her basically nothing about our life and relationship. She even asked in the one email If he and lived together and how often he saw the baby. All the emails and texts are signed with xoxo on both sides. I only know this much info because I got suspicious and checked his phone and emails. He has told me they keep in touch from time to time, but I just don't like it. I cannot approach him as I know I was in the wrong by going through his emails ... I just had a gut feeling that something wasn't right. I am SO angry right now. I just saw the emails a few minutes ago and I don't know if I am over reacting or what. The only semi comforting thing is we live about 700 miles from her. I know he would not appreciate this if I was the one doing what he is doing. Advice?