It looks like the only strange thing, whatever it was, happened yesterday, alone. Now, after a bit more time than I expected (more than an hour), the traffic seems back to normal. I'm sorry for the noise.
One of my friends got a ransomware and, even if I am on Linux and I take care when browsing, with such a proof beside me I thought I'd secure myself a bit, so I got ufw. I am running a middle relay, and so far it went just fine with ~2.5MB bandwidth, even if only in the daytime, morning to evening (laptop...; I do plan in getting a RaspberyPi in the future).
I followed this page for setting up ufw with tor: https://help.ubuntu.com/community/TorDedicatedUser , even if I have Archlinux. I didn't use all in there, only the part right below "Traffic redirection", with ufw in the command line. Now, if I see the relay with 25kB, it's a miracle.
So, obviously, I did something wrong. Is there a page with proper explanations for dummies like me? If it helps, my net comes from a router that handles cable TV and phone, as well.
Vlad