There are a few possible causes of this. You probably haven't been downvoted 15 times (or cast 30 downvotes yourself) without knowing, so I'll skip those "possible" causes and cover more likely ones instead.
###Serial Upvoting Reversal
If you got many upvotes from the same person in a short time, then the system will identify this as serial upvoting and remove the reputation associated with it. If this happened, you should see serial upvoting reversed on your reputation page. (I cannot see that, even if it is there--most negative reputation changes to your profile can only be seen by you, and moderators.)
###Unaccepts and Unupvotes
After a user votes on a post, their vote is locked in after a short time. But they can change it if the post has been edited. Furthermore, there is no lock-in of accepts. So if the creator of a question marked your answer as accepted, they can unaccept it at any time (causing you to lose the 15 reputation you gained from your answer being accepted).
Two unaccepts would result in a loss of 30 reputation. Or 3 unupvotes on answers.
###Deletion
If you've delete a question or answer you've posted, then you'll lose any reputation (both positive and negative) associated with it. This also happens if your posts are deleted by someone else, or automatically by the system. For example, if you had an upvoted question that was closed, it may have been subsequently deleted.
One or more questions you answered (and earned reputation from) may have been deleted. When a question is deleted, all its answers are deleted too (though they will come back automatically if the question is undeleted). Alternatively, your answer may itself have been deleted. You can delete your own answers, but sometimes answers are deleted by moderators, too.
A common reason moderators delete some upvoted answers is when a question is marked as a duplicate. If your answer doesn't provide any information beyond what is provided by answers in the master question, it may be deleted rather than merged. Of course, a question can be closed as a duplicate with removing or merging any answers. However, sometimes answers are deleted so that the question that's been closed as a duplicate will automatically redirect to the master question.
###Migration
If you asked or answered a question that was migrated to another Stack Exchange site, you lose any reputation associated with it here, and gain that reputation on the target site.
(The reputation becomes associated with your account there only if that account is associated with your account here. If you have an account but it's not associated, you'll only gain the reputation on the target site by associating it. If you don't have an account, you'll gain the reputation on the target site by registering there and associating the account there with your AU account here. This can be accomplished by using the same OpenID to log in to both accounts. Or you can use different OpenIDs but make sure they're all listed in your single network profile.)
###How to find out?
Usually, you can determine the reason for a loss of reputation by checking your reputation page. Often it's necessary to check the box at the bottom labeled show removed posts. That will probably show what happened to cause the loss of rep.
If you still cannot determine the cause of your recent rep loss by looking at your reputation page with show removed posts checked, please update your question to let us know.
If that doesn't reveal it, you can check your reputation audit at http://askubuntu.com/reputation. (This URL shows different information for each user.)
