[tor-relays] traffic pattern indicates MITM?

For four days the BW graph and usage table of my bridge indicated noticeable traffic and then the graph dropped precipitously to near zero & the table shows no recent usage. Vidalia/tor are working fine. I did try rebooting my router-modem gateway last night, but as far as I could monitor it, the traffic has still been near zero (maybe a few KB every hour or so). If I'm reading it correctly, onionoo is showing that the bridge is still published.

I'm not complaining about this pattern, but wondering if the abrupt drop-off in traffic for a still-published bridge is diagnostic of MITM activity. Maybe there's a way to further monitor my bridge from within the network? If MITM is suspected, should I shut down the bridge for a day or so?