So I was on Route 3 (just after Pewter City), and I was battling trainers to level up. I had just logged in for the night and went straight to the NPC YoungsterCalvin near the very start of route 3. I was using my nidoran male (lvl 10) against his Spearow (lvl 13). I was down to half hp, and used double kick. After my attack hit and took away some of the Spearow's hp, my pokemon healed itself to full health. It did not have any berries, nor did I use any healing items at the exact moment. I do not know if this can be recreated. I should mention I just started my journey as well, so I have all low-leveled Pokémon.

It's a visual bug, no biggie.