Well i checked the pokemon wiki and hitmontop is in the IQ group C
and i checked the groups abilitys with the IQ and non have anything to do with reflecting damage.
Which brings me to the obvious conclusion that in a previous turn hitmontop used the move (i forgot what its called) that reflects the damage you do to it for a few turns unless you knocked it out in that hit. It may be counter.
But correct me if im wrong but dont all newly recruited pokemon come with minimum Iq abilitys.
Refrains from attacks that would be blocked by walls or allies.
Can use the Explorer Badge to leave the dungeon. Cannot be turned off.
Can catch a Throwing Item and keep it for its own.
Can use and throw its held item.
Uses only moves, not regular attacks.
Refrains from moves that inflict a status change that the foe already has.
Does not attack allies, even when confused or in cowering status.
Adds 10 to max HP. Cannot be turned off.
Raises the critical-hit rate of type-effective moves.
Throwing items deal 1.5 times the damage. Does not work with Geo Pebbles and Gravelerocks.
Restores more Belly stat when eating food. Sometimes avoids getting status problems from Grimy Food.
Pokémon is immune to nightmare, sleep, yawning and napping.
Raises Accuracy by 1 but decreases Evasion by 1.
Raises Attack and Special Attack by 1 on the turn after a missed attack.
Prioritizes attack foes with a higher experience point payout.
Raises both Attack stats by 1 each, but decreases both Defense stats by 1 each.
Lowers Defense by 1, and sometimes sends the damage taken back to the foe if the foe is adjacent.
Avoids entering lava.
Increases multiplier effect of type compatibility.
Raises Evasion by 2 when HP is below 25%.
Sometimes strikes again following an attack.
Sometimes causes attacking foe to cringe.
Unleashes charged attacks in one turn, while draining 2 PP.
Throwing Items penetrate Pokémon and walls and continues flying.
so i took a second look and i found the iq skill extra striker which may have been your problem it allows them to strike twice sometimes but other than that i dont have a clue what is happening i thing its this skill it has crippled me in the past.