I'm going to assume that you're either in a faction by yourself, or you're the only person with suspiciously low power in that time frame.
Currently, power drops at a rate of 1 per day, so if you were at full power, it would take you a full week to drop down to three power.
The problem with checking things that happened several days ago is that there's nothing really to check. The server log keeps a record of the last six hours on the server, but things that happened several days ago are usually impossible to check on.
It's always possible that someone hacked your account and did something funky with your faction. It's also possible that your faction power dropped faster than intended. Unfortunately, without anyway to verify what happened one way or the other, there's not too much we can do.