Bandwidth Problem in our office

We have 10 computers on our office and we are consuming 23GB or more last week where mostly we are just using the internet for emails and browsing. We had an IP replacement as requested by our ISP, but still. As I opened my Norton Security Network map, it showed me our local computers plus hundreds of devices on our network that I don’t know which runs on a 10.1.xx.xx range where our IP addresses are on the 192.168.xx.xx range. Does this mean we are being hacked?