Many times my reputation is decreased
I found writing: Serial upvoting reversed
What does it mean?! what the wrong i did.
This issue repeated 3 times till now
This means a user has been upvoting you many times in a row, and it appears that they are unfairly trying to give you reputation. You can ask a moderator in chat to ask the user to stop.
With that said, you are not in any fault unless you controlled the account that upvoted you serially, or asked for those upvotes(solicited them). If you have done so, please just stop now, as no bad action has been taken against you yet.