It is very unlikely that your seed has been hacked or cracked by someone if you did not make a mistake. My assumption is that you must have used fake metamask which was discussed a lot yesterday. This is clearly your negligence.
On the other hand, somehow I do not really believe in metamask because its use is only installed on the browser. Which could be a bad person copying it to take your keylogger whether it's seed, mnemonic, and privkey. Try to be more careful next time using a wallet other than metamask like Trustwallet for mobile devices and use MEW or hardware wallet if you are using a PC.