Thanks you for your answers !
Well, first of all : yes, it was (I'm absolutely sure about that) a tutorial game. The one right when you finish the elementary tutorials, the short 10 planets campaign with many advices. Those were really helpful and really good to make me aware of the next danger, but they never prepared me to something close to that.
I feel that even with a top-notch preparation, I couldn't have beat that. Even with all my defenses, a thousand was left, rushing to my homeworld, meeting almost no resistance.
It is quite funny because the rest of the game was a breeze : almost no difficulty at all (of course, it's tutorial), and then, bam. I'm still laughing a bit when thinking of it.
To answer you Keith, I'm not really sure what ships they were, I think those were Tanks, Missile Launchers... In short, a reverted big bang right in my face.
However, I'm not sure that I didn't destroy any Core stuff if the battle right when I came in. I just X+G right in the front so maybe I destroyed something I shouldn't have to. The problem is, I don't know what I did wrong.