Chemical sunscreens work by absorbing into your skin. Then when the sunlight goes into your skin it changes the chemical make-up of the sunscreen. (the chemical sunscreen "absorbs" the sunlight and the energy from the sunlight changes the chemical)
Please try an inert sunblock (which literally blocks the sunlight before it ever enters your flesh, reflects it right off the surface) or long pants/sleeves/hat until you get in to see the dermatologist.
It sounds like you might be reacting to the secondary structure of the chemical sunscreens in your L'Oreal. (a sunburn or a reaction to the primary structure would probably not disappear as soon as you left the sun, but only your dermatologist could tell you what is really going on)
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