Preheader Text issue - Text from my email is being appended to the end of my Preheader text

When I add my Preheader text, it looks great ,until I send my email. Constant Contact keeps appending text from my email to the end of my preheader text. The preheader text looks really bad like this. I receive a lot of email newsletters and all the one's I receive have just their sentence. It's nice and clean. How do I stop Constant Contact from adding text to the end of my preheader text? Thanks!