What is a bright pink square pill with DP on both sides?

It's known as "diphenhydramine," which has many brand names. The most common is "Benadryl." Benadryl is an antihistamine, which counters the effects of histamine. Histamine causes itching, sneezing, watery eyes, and a runny nose (popular during allergy season).

My expert advice is to always throw a pill away that you find laying around. Never take medicine unless it's in a bottle in YOUR home.