As other users already stated, Splash does not deal damage.
I'm not sure, but I think I somewhere read something about a bug where, when you re-arrange the attacks of a Pokemon, the game doesn't get it and re-orders only the attacks shown and the messages, but not the effects. So, you could for example get a Protect that is called Relic Song both on your screen and in its message, but which will still have the Protect animation and say "insert_Pokemon_name_here protects itself!" (Not sure whether there are Pokemon with both in the moveset, but never mind.)
So that's what might have happened to you, you used a Tackle the game called Splash.
Another idea: Did you play an original game, or a patched one? Because if you're using patches, the hacker might of course have changed the code for Splash so it occasionally will deal damage.
If it does come from a hack, you should remove that hack, because a hack that actually wastes space for modifying Splash is way too low quality for being worth it. Only the truly brilliant hacks are hacks you should load, and even for them you should think at least twice, because of the risks. I'm both talking about load errors breaking your game for good, and that the game developers could go after you for modifying their game - a game is copyrighted material, after all.