I don't think pts will decrease by themselves due to inactive.
How do you check who had up you?
Click the "User CP" ( it's beside FAQ )
or click below link:
http://sbfsg.org/usercp.php
Check your history in User CP, see who had up you and "who" had deduct your points.
Normally people who zap you, they don't put his/her name.
Inside User CP, there are 3 columns,
1) New Subscribed Threads
2) Latest Reputation Given, right side can see your Reputation Power.
3) Latest Reputation Received, right side is your point(s) total
Under Latest Reputation Received,
You will see 3 color highlight, gray, green and red.
Gray = Someone have no power and up you.
Green = Someone have power and up you.
Red = Someone had deduct you points.
The system don't tell you how many points they up/zap you.
So you have to see the comment they give you, get the user name and search the person see how many power he have and you get how many points.
If that person 3 year ago have power 5 and up you 5pts, now he have power 8 but your points don't increase.