I had 5518 re, but when I refreshed the page, it suddenly dropped to 5508. How?
It is probably because someone who previously up voted one of your answers subsequently removed the up vote.
This is also true for Accepted answers. If your answer is marked as accepted you will receive +15. However, if the OP changes their mind and marks a different answer as accepted then your Rep score will decrease by 15.
There isn't really any way to know which questions/answers these events occur on - unless you memorise all of your answers and upvotes.
See this comment from Grace Note
from this question I posted a while ago