Is my husband interested in his secretary?

My husband and I have been together for 16 years now and of course we have been thou lots of things in that time. We have never either of use really felt insecure to the point of where we felt one of use could be considering being unfaithful. Of course he has always worked and been around other women at his place of business or socially and I have never once until now felt that something wasn't right. From the very frist encounter that I had with his new secretary my gut feeling was that things weren't right. I have tried telling him that I had these feelings about him and her. He said that he only loved and wanted me and wanted me to feel better about her he asked me what he could do to make me feel better ( I WANTED TO SAY FIRE HER BUT KNEW THAT WASN'T GOING TO HAPPEN). Well these is the part where it gets even more concerning just last week I checked his phone records and seen that they have been texting but when I looked at his phone for the messages he had deleted them. He said he only did that because he knew that if I seen any text between them I would be mad but that they where nothing more then work related and there was nothing more. THEN AS I REALLY LOOKED AT HIS FB I FOUND THAT HE WAS HER FB FRIEND WHICH HAS NOTHING TO DO WITH WORK AND HE HAD EVEN SENT HER A MESSAGE THERE AS WELL THAT HAS BEEN DELETED AS WEL.L. NOW IM SO HURT AND EVEN MORE WORRIED. I NEED TO FIND AWAY TO FIND OUT WHAT'S GOING ON WITH THEM BEFORE I GO CRAZY. IM JUST NOT SURE HOW TO FIND OUT ANYMORE THEY ARE TOGETHER EVERY DAY AT WORK AND IM NOT THERE TO SEE WHAT'S GOING ON. HELP ME