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.
edit:
Course Checker
Base Skill
Refrains from attacks that would be blocked by walls or allies.
Escapist
Base Skill
Can use the Explorer Badge to leave the dungeon. Cannot be turned off.
Item Catcher
Base Skill
Can catch a Throwing Item and keep it for its own.
Item Master
Base Skill
Can use and throw its held item.
Exclusive Move-User
Base Skill
Uses only moves, not regular attacks.
Status Checker
25 (☆0.5)
Refrains from moves that inflict a status change that the foe already has.
Nontraitor
40 (☆0.5)
Does not attack allies, even when confused or in cowering status.
Brick-Tough
100 (☆1)
Adds 10 to max HP. Cannot be turned off.
Type-Advantage Master
105 (☆1)
Raises the critical-hit rate of type-effective moves.
Power Pitcher
130 (☆1.5)
Throwing items deal 1.5 times the damage. Does not work with Geo Pebbles and Gravelerocks.
Survivalist
140 (☆1.5)
Restores more Belly stat when eating food. Sometimes avoids getting status problems from Grimy Food.
Nonsleeper
160 (☆2)
Pokémon is immune to nightmare, sleep, yawning and napping.
Concentrator
185 (☆2)
Raises Accuracy by 1 but decreases Evasion by 1.
Practice Swinger
195 (☆2)
Raises Attack and Special Attack by 1 on the turn after a missed attack.
Exp. Go-Getter
200 (☆2)
Prioritizes attack foes with a higher experience point payout.
Aggressor
220 (☆2.5)
Raises both Attack stats by 1 each, but decreases both Defense stats by 1 each.
Counter Basher
280 (☆3)
Lowers Defense by 1, and sometimes sends the damage taken back to the foe if the foe is adjacent.
Lava Evader
300 (☆3)
Avoids entering lava.
Erratic Player
340 (☆3.5)
Increases multiplier effect of type compatibility.
Clutch Performer
360 (☆4)
Raises Evasion by 2 when HP is below 25%.
Extra Striker
520 (☆5.5)
Sometimes strikes again following an attack.
Intimidator
560 (☆6)
Sometimes causes attacking foe to cringe.
No-Charger
570 (☆6)
Unleashes charged attacks in one turn, while draining 2 PP.
Pierce Hurler
850 (☆8.5)
Throwing Items penetrate Pokémon and walls and continues flying.
-bulbapedia
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.
