I got upvotes today on 20 different answers, all very well deserved, but 19 of them came within a span of 15 minutes. Assuming they were all from one account, does that meet the test for "serial voting", and are the points going to be taken away?
Yes, based on your description, it's almost certain that you've been the recipient/victim of a serial upvoter. My guess would be that given your proclivity for solving story-identification questions, someone you've helped has decided to reward you.
Unfortunately, it's very likely that those rep points will be nullified when the serial-upvoter script kicks in and removes them.
If the voting fraud is in the progress of happening or just happened recently, don't worry about it. You should wait at least 24 hours after noticing before becoming concerned. The system should detect it and reverse it for you. Please do not try to get help on this issue on meta or by flagging for a moderator. All they will do is tell you to wait for the voting fraud script to run (they won't run it for you just this once).
Why don't I get to keep the reputation?
The reputation was removed because it is not proper behavior and it is not allowed. The votes were completely invalidated by the system and thus the reputation gain from them was also invalidated. The only way you can gain this reputation back is to go post some more and get some legitimate upvotes on those posts.
Reversals trigger a reputation recalculation.
Whenever serial voting gets reversed, all days involved in the serial voting will be recalculated as if the serial votes had never happened, and any reputation you might have lost due to those serial votes hitting the reputation cap will be given to you at that time. Serial voters will not prevent you from gaining reputation that you deserve.
The exact mechanics for the serial vote check is kept private, for obvious reasons, but your situation sounds like it will get caught:
Every night around 03:00 UTC, a voting fraud detection script is run that looks for patterns such as these. It basically looks for users who have upvoted another user many times. The number is fairly low within a given amount of time (the exact mechanics are kept secret)
If you get caught, you'll know by 3:00 UTC tomorrow morning, and your rep tab will show it.
As far as the upvotes being deserved: the reason for the serial vote check is essentially that the votes came in too fast for the voter to have actually read your answers. That might or might not be true in every case, but in general, the fraud detector is tuned to hit the right balance. If those 20 upvotes came in so quickly that the voter was voting for you and not your answer, deserved or not, they'll get backed out.