I'm not sure what's causing it, but suddenly the
systemd process just started using about 150% CPU (according to my iStat menu). I checked in Activity Monitor and it said the same. I checked the CPU temperature and it was almost 100℃ so I immediately shut down. I turned it back on and the same thing happened, so I shut down again and, after trying a couple more times, logged into the guest account.
So now I'm in the guest account, everything's fine, but obviously I can't stay here forever. I can get some additional information if you need it, but I don't really want to go back into my user unless it's necessary (100℃ is very high).
(p.s. I'm running High Sierra 10.13.4 Beta. I'm tempted to get off the Beta version, but I'm not sure how)