And of course, the downloaded wallet is from official Electrum.org.
I´m pretty sure that my Win7 Dell is very well protected... and it was powered on, I was controlling it remotely from my phone. Well, I assume that the most probable case is the bug-thing. Good to know...
Never ever gonna use Electrum anymore.
This is common behavior for users who downloaded an infected version of the wallet (receive coins and immediately have them sent to an unknown address - owned by the hacker), so you probably did something wrong somewhere.
There is also the fact that you were controlling it remotely. So, a lot of possibilities here.
My recommendation? Do a full reinstall of your OS and create a brand new wallet. Both are probably compromised.
Never ever gonna use Electrum anymore.
It's not Electrum's fault that you haven't kept it up-to-date (now it notifies users about available updates). It looks like your computer has been infected with some kind of malware so I would recommend scanning it with Malwarebytes. Consider buying a hardware wallet. Even if your computer is compromised, your funds won't be spent without your knowledge (unless someone finds your seed or there is a bug which requires a physical access to the device).
OK. Well thank you all guys for the tips and advices. Will do...