Contact Dermatitis or Irritant Dermatitis: This is a common condition that can occur after exposure to an allergen or irritant, leading to fluid-filled blisters. Given the age and presentation, this is a plausible explanation, especially if the girl has recently started using new soaps, lotions, or has been exposed to plants like poison ivy.

Insect Bites: Mosquitoes, bed bugs, or other insects can cause small, fluid-filled blisters. The reaction to the bite can vary, and some individuals may develop significant swelling or blistering.
Friction Blisters: If the girl has been engaging in new physical activities or wearing clothing that causes friction on her thigh, friction blisters could be a likely cause.
Folliculitis: An inflammation of the hair follicles, which can be caused by bacterial or fungal infections, leading to pus-filled or fluid-filled bumps that might resemble blisters.

Cellulitis: Although less likely to present with distinct blisters, cellulitis is a bacterial infection of the skin that requires prompt antibiotic treatment. It can start as a small, red, swollen area and can be serious if not treated.
Erythema Multiforme: A skin condition characterized by target lesions, which can sometimes appear as blisters. It can be triggered by infections or medications and, in rare cases, can be a sign of a more serious condition.
Staphylococcal Scalded Skin Syndrome (SSSS): Caused by staphylococcal toxins, SSSS leads to widespread blistering and peeling of the skin. It's more common in younger children but can occur in adolescents.

Bullous Pemphigoid: An autoimmune disease that causes blistering of the skin. It's rare in adolescents and more commonly seen in older adults.
Pemphigus Vulgaris: Another autoimmune disease that leads to blistering of the skin and mucous membranes. It's rare and usually starts in the mouth before affecting the skin.
Epidermolysis Bullosa: A group of genetic conditions that result in fragile skin and blisters. Given the age of onset and the localized nature of the blisters in this case, it's less likely but should be considered if there's a family history or other suggestive findings.