Yes, that transaction was done by me only. And I checked the source, which is why I used my private key there when I was online. I believe I should not receive any coins over that address anymore but should keep it to sign messages.
That's good news. I always keep the private keys to all my old addresses for that reason. As was already pointed out if a key is compromised then potentially someone else could also sign a message but it is an unlikely way to hijack a bitcointalk account.
I'll be opening a thread in Meta and telling people about the situation, so as to stop a "possible hack" of my account.
Can you answer my queries told in the same comment?
I'm not sure exactly which queries you mean. If it is about signing messages using Segwit addresses then the answer is not to use P2SH-P2WPKH addresses (the ones that start with 3) but to use bech32 native Segwit addresses starting bc1.
I've been using this on Electrum since mid-November. With Electrum it is easy to use multiple wallets so I've kept my legacy address wallet so I can still receive payments from sites that don't yet support withdrawals to bech32 addresses. Hopefully, after Core 0.16.0 is released then everyone will start to introduce support and I can retire the legacy wallet.
Thanks to you and everyone who helped me out, I've finally downloaded Electrum wallet, saved my key and generated SegWit addresses as well.
Don't know how will I be able to sign messages from these bech32 addresses, though. But I am all fine now and hope that it will be beneficial for me as well as the senders/receivers from/to whom I get/send Bitcoins. Will sign a message and post a thread in Meta in a few moments.