i was playing the free weekend and when it was over it says i am 15. but in the pick champion menu it says im 19. i lvled up at 15 and got 2 points instead of one and now it says im at lvl 15? what happend

Are you by chance playing with the free trial? It caps your character at Level 15.

yes but im in the city not the infested one with the bugs but it said i am 19 at the char screen and now that i lvled up at 15 i stay 15 but still strong enofe to kill lvl 18

Buy the game to keep leveling beyond 15. As PatchworkCat said, you can't get any higher then level 15 with a trial account.
I'm relatively sure you'll get your levels back when you've bought the game..

Can anyone confirm that your XP does indeed catch up if you purchase the game? I'm in a similar predicament. Thanks!

Yes, you do get bumped up to your proper level when you get the full game.