It would make a lot of women's skin crawl / there are plenty of women who would feel the same way as you.
You do describe it as a casual relationship at that time.
In terms of whether or not you overreacted to finding out about this, and him still talking to her regularly...the only person who can accurately answer this for you, is you. Others can give you how they would react, but we humans can vary widely in how we feel about certain issues. 'Group Consensus' follows the Bell Curve (look it up), and is no guarantee of answering this accurately for you.
I would recommend asking yourself why you reacted the way you did, identifying your reasons, then asking yourself how valid the reasons are to you. From there its much easier to answer for yourself 'did I overreact'...or not.
One of things forums are good for is giving you different perspectives to think about (it's sometimes difficult to get past thinking on our fears / inseccurities).