Do you have an ethernet cable plugged into all the network ports (usually only one, but since you didn't specify motherboard, there could be more than one)?
Some ethernet adapters will run at 100Mbit when the PC is off to support WOL (Wake On LAN). During the initialization they disconnect so that they can auto-negotiate the speed. It is also quite possible you are seeing that. Which is not an issue.
If your network port is functioning, I would ignore it.