My site has been consistently maintaining the same rank in a search. But after I release several URLs with tags used for tracking, my rank started dropping. I read somewhere that if you have a ? in the URL it will not get read properly by the spiders. Any idea how a search engine could be using these new URLs in a negative way?

For example, my original url was:
www.mydomain.com

The new URLs that I had available were:
www.mydomain.com/?key=blue+shoes

Thanks in advance.