Traffic- how much is too much

The linode by default is capped at 50mbps. With technical justification, you can get that raised with a support ticket.

That is a lot of bandwidth, especially if you don't know why. You'll have to figure out what it is doing and why before we can tell you if that is a lot. If it is doing what it is supposed to be doing, it's sort of up to you to decide what is "too much"

You used up ~86GB in the past day, which if you're on the smallest sized linode, is almost half your monthly allotment in one day.

A tool like iftop while the problem is happening can tell you what connection is using the bandwidth, or what process/port. Otherwise, check your web server log files.

For the life of me I cannot figure out where that all went, though I'm not great at reading log files. It may have been a misbehaving bot, but Google analytice knows nothing of it, nor does Cloudflare.

I'd suggest starting out by running a:

du -sh /home/*

(or wherever you store web files) to see if there are any especially large files capable of using large amounts of bandwidth quickly.

Then search through the access_log files in /var/log/httpd/. If you aren't logging, the best you can do it at this point is start (http://httpd.apache.org/docs/1.3/logs.html).

If it's a sudden spike, it's likely not just simply an organic traffic surge for low-volume content that would just disappear the next day.