It’s a skin condition called phytophotodermatitis, a reaction caused by the chemicals in some fruits and plants (most notably, limes, lemons, and celery) that make your skin hypersensitive to the sun.
Here is part of an article on the topic with additional information from Woman's Health Magazine:
The Scary Signs
Within a day or two of exposure, you’ll notice some redness and irritation, which may include blistering in more severe cases, says Green. But the most noticeable result is hyperpigmentation, or brown spots, which appear up to a week later and can last for several months. This reaction can show up on any spot on your body that was exposed to both the chemicals and the sun. “Usually the cue is linear streaks from where they poured or spilled lime juice,” says Green. “Or if they were squeezing lemon on their hair, they may see brown streaks on their faces.”
How to Deal
So what happens if you spot this reaction post-vacation? If you have a severe burn with blistering, you may want to see a doctor for a prescription-strength hydrocortisone cream, says Green. In some cases, your MD may also provide a bleaching agent for the hyperpigmentation, though it’ll eventually fade on its own. If you’re just sporting some sunspots, you can skip the doctor and use an OTC hydrocortisone cream for relief. But don’t panic—the reaction isn’t dangerous, and it goes away in time, says Green.
To avoid it from happening in the first place, take precautions when handling fruits outside (and note that these chemicals are also found in parsley, parsnips, dill, and various other fruits and plants). “Don’t mix drinks while in the sun, and wash hands immediately so there are no traces of chemicals on them,” says Green. You can even use gloves to be extra careful, and you should also make sure to clean up any spills to keep chemicals from finding their way to your arms or legs. And if you’re enjoying a cocktail at a cookout or pool party, consider skipping the lime wedge in your Corona and swapping that fresh margarita for a piña colada—your skin will thank you.