XP gains messed up in a normal draft pick game

Just got done with a 46 minute game in which every single person was either lvl 17 or 18, but I was stuck at 14 for the last 15-20 minutes. I would solo clear lanes and get 1-2xp per minion kill. Even when we Aced the enemy team for the win I was right there, alive, got 5 assists, and my xp bar barely moved.