Could this be a reason my site dropped rankings?

I'll try to make a long story short. I started my wedding veil site in 2006 and with good link building and seo help it got in the top 5 rankings in Google and maintained those rankings up until about 5 months ago.

When I first built this site, the shopping cart I used had dynamic URLS - example
http://www.everafterbridalveils.com/shop/?cat=7&shop=1, however, I still managed to get in the top 5 positions of Google for most all my keywords, and maintain those rankings as I've previously stated. I know the site wasn't perfect and I have an index.html still showing, but this never affected my site.

About 7 months ago, the person whom I host my site with decided he was going to change all the URLs on everyone's sites that use his shopping cart to be more search engine friendly meaning a page like this:
http://www.everafterbridalveils.com/shop/?cat=7&shop=1 would now read http://www.everafterbridalveils.com/category/Bridal-Jewelry-7 He did NOT delete the old ULRS as they are still around, but he also did NOT redirect the old URLS to the new ones. I also was not given a choice in this matter. There is like around 500+ of these links out there and I can't even retrieve them all to redirect. I was concerned at what would happen to my rankings as I thought Google might see this as duplicate content.

Sure enough, about a month or so after he did this my rankings began to go from 4th or 5th position for "wedding veils" to about 7th or 8th position. It slowly has now crept down to 10th position and sometimes down to Page 2. The term "bridal veils" maintained for a while, but now it's sunk down to 10th position on that term too. However, for the term "Tiaras" I still rank at 4th to 5th position like I have. I asked the guy who hosts the site and he claims the changes he made has nothing to do with it and Google probably wasn't seeing this as duplicate content. He told me to use Google tools to check - which I tried to find but couldn't in the webmaster tools. I used other duplicate content checkers and got mixed results. Do you think this could be the reason my rankings slowly declined? My links have remained the same in fact a few more blogs have even added my site to them.

changing URLs almost certainly had everything to do the the drop. The guy's a retard. Take care of the 301s and the ranks may come back.

Thank you for answering my question. I thought this was probably the reason for the rankings. The guy told me I could set in my robots.txt that specific old style url to not be indexed. Is this a bad idea or is it better just to forget that and do the 301 redirects?

Stop listening to that guy.