It can only happen really on one occasion: if a post was taken down for any sort of reason. If your post was taken down, weither it was the bug, a site moderator, or you took it down, you will loose all the XP you've gained from that post.
One of three things has happened: 1. You removed some submissions yourself 2. A mod removed one or more of your submissions. Check your PMs for a PM from a mod 3. One or more submissions was lost. We've been having problems with some submissions disappearing from our site. Look through your list and see if you can see anything missing