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There are lots of variables that can affect the sound pressure at your ears. +P means that the round is loaded to a higher chamber pressure, for increased muzzle velocity. This increased chamber pressure ends up creating a more pronounced level of sound pressure, all else being equal, than standard pressure rounds. Additionally, shorter barrels (like in many carry guns) mean greater flash and report than longer barrels due to propellant continuing to burn and create pressure closer to, or beyond, the muzzle. Though the OPs is not one, revolvers also have the barrel-cylinder gap that gasses escape from. Ported barrels or compensators also strip high-pressure gasses off the barrel. All this is just related to the cartridge and gun.

Beyond the gun, there are environmental factors that can either boost or help attenuate the sound pressure to your ears. The OP mentioned proximity to his truck, which is something that would reflect the sound rather than letting it continue. The type of surfaces around you -- concrete versus dirt, walls versus trees, etc... also play a part.

Hopefully this incident will not leave the OP with lasting damage.