Insect Bites: The appearance of small round papulae after camping, especially on exposed areas like the lower extremity, is highly suggestive of insect bites. The outdoor exposure increases the likelihood of encountering biting insects.

Contact Dermatitis: This could be due to contact with plants or other allergens during camping. The reaction might manifest as small papulae.
Folliculitis: An inflammation of the hair follicles, which could be caused by friction from clothing, sweat, or bacterial infection, common in warm and moist environments like those encountered during camping.
Eczema (Atopic Dermatitis): Though less likely to be directly caused by camping, pre-existing eczema could be exacerbated by environmental factors encountered during the trip.

Tick-Borne Illnesses (e.g., Lyme Disease): While the primary concern is the distinctive "bull's-eye" rash, some tick bites may present with smaller papules. Given the potentially severe consequences of untreated Lyme disease, it's crucial to consider this diagnosis.
Chigger Bites: These can cause intense itching and small papules. Though not usually dangerous, they can be very uncomfortable and are a common issue in outdoor settings.

Leishmaniasis: A parasitic disease spread by sandflies, which could be encountered in certain camping environments. It presents with skin lesions that can start as small papules.
Schistosomiasis (Swimmer's Itch): Caused by a parasitic worm infection, typically acquired through contact with contaminated water. It can present with small, itchy papules.
Pityriasis Rosea: A skin condition that can start with a single "herald patch" followed by smaller papules, though its direct link to camping is less clear.