I noticed my rep went down by -70 points, and is the first have heard of that, what does it mean?
Edit: I am a StackOverflow user and has never come across it before!
The system basically detected a series of upvotes on your account within a very short timeframe all from a single user. This is not an assessment of your behavior, but rather the behavior of a "fan" that you may have acquired. Similar to targeting a specific user for serial downvoting, serial upvoting is reversed automatically by a script that runs about once a day (IIRC).
The idea here is that it's intended to help prevent sockpuppeting (note that I am not saying that's what you are doing, simply that it is the purpose of the algorithm).
There are also some discussions on MSO that you may find useful. There are more than these few questions, but I think they are the most relevant: