I'm so worried, please i need your help!?

Before me and my husband got married, there was a lady who use to call at home to speak to my husband. So one day i asked my husband jokingly if he has ever dated the lady, he refused and said she was only a friend. He had even dined with this lady while he was away for work in London and while we were together. All i knew was she is his friend. They met during one of his holidays before we ever met each other. One day i was in the study looking for some doents when my eyes fell on a plastic bad tied in a bundle. I pick it up, untied it to fine out what it was, It was a collection of letters and the first one i could lay my hands on was a letter from this lady, telling him how much she loved and missed him. In short, love declaration. I was shocked, and i thought why would he lie to me especially as its his past? I confronted him, and all he said was that, its the past and its nothing to worry about. I'm angry and worried that he lies about something so small..........please help!