Traffic when not online:
This is a special drawback of almost all P2P networks. As you start aMule, it connects to a server, and then aMule sends it's IP and files list to the server. Then the server can give your IP to clients as reply for searches. Clients connect to you, and they might even save your IP+port as a source for a file. When you close aMule, they will not know about it (as queue is connectionless), and keep sending file requests until they mark your IP as dead source and remove from their list. These tries cause you a falling traffic from ed2k clients just after you closed your aMule.
This traffic lowers by time, say it's reduced to half after an hour, to 1/4 fter two hours, and so on, until it gets to almost nothing.
You will find it the same with every P2P network, that doesn't need constant connections to everyone who knows about you - ed2k, gnutella, G2, FreeNet, just to mention some.