The "monitor traffic" command is only going to show you traffic to/from the routing engine.
In order to monitor transit traffic, you'll need to configure packet capture filters. This KB has the steps to get that going.
As I mentioned... lots of ways to do this. If you use the on-device packet captures, I'd recommend you transfer the files off to a workstation so they can be inspected with Wireshark or something more friendly.
Capturing packets and looking at the captures in tcpdump or Wireshark is a pretty manual, tedious process if you're looking to see where large traffic spikes are coming from. I'd really recommend letting some other piece of software do the heavy lifting for you, like ntop. It will give you statistics on how much traffic of what type is flowing, how fast it's making new connections, etc.