these are from the top of my mind.
1. How many machines do you have? If it's not significant bandwidth that you have(not over 100Mbit/s) you could monitor the network with wireshark. Adding filters to sniffer could help you peal off the traffic that you consider more likely not to be relevant.
2. I don't want to frighten you, but if you truly haven't been doing anything that you know could produce that kind of traffic, then this sudden "verbosity" of your network could be indication of presence of bot-nets. They usually have exact time when they should contact the server. This peak might mean they all rushed to contact their "boss".
You could check the presence of bot-nets with NIDS such as Snort, though I think it should be last to do, because installation might be "kind of an adventure"(as someone said), and it is certainly killing a fly with a gun, if it's only a service that somehow cut loose and gone wild.
If you decide to run Snort, try downloading Insta-Snorby, because it suppose to work out of the box(at least producers say so). It's free, too...
Hope this helped.