First, I would would like to state that I have also been in hacked lobbies. I didn't know it at first until some time when I would see things that don't make sense.
ie.
Ridiculous point accumulation for simple things like kills, Killstreaks, Killstreak kills, etc...
The point I'm making is that you're not alone. As long as you realize there's something going on and you leave A.S.A.P., then you're okay.
Second, you have to report all players that you can. What I usually do is press Select to see the list of players and I take a pitcure with my phone. Then, I check on them on Elite and I see who accumulated the most points. In your case, I would have checked which gamer kept on playing and kept accumulating the most points. There's a very good chance that gamer was the hacker.
I report hackers/cheaters by email to report@infinityward.com; Tweet to IWEnforcers and ATVIAssist.
Lastly, the points and XP's you got cannot be deducted. Your honesty is appreciated, but I don't think they can do anything other than reset you or ban you. If you did nothing wrong, you have nothing to worry about.
Of course, they're still going to check if your statement is valid; as in you played the game for a reasonable amount of time for you to figure out that things were too good to be true.
