Insect Bites: The localized, itchy, and bumpy nature of the rash, along with the patient's recent travel to Colorado, suggests insect bites as the most likely diagnosis. The lack of relief from diphenhydramine could be due to the severity of the reaction or the type of insect bite.

Contact Dermatitis: Although the patient denies new soaps, lotions, or medications, contact dermatitis remains a possibility, especially if the patient came into contact with an allergen or irritant during their trip.
Eczema (Atopic Dermatitis): The itchy and bumpy nature of the rash could be indicative of eczema, particularly if the patient has a history of atopic diseases.
Folliculitis: Given the location on the calf and the description of the rash, folliculitis (inflammation of the hair follicles) is a possible diagnosis, especially if the patient has been wearing tight clothing or has been sweating excessively.

Cellulitis: Although less likely given the absence of systemic symptoms like fever, cellulitis is a serious condition that requires prompt antibiotic treatment. The presence of significant redness, swelling, and warmth in the affected area would increase the suspicion for cellulitis.
Deep Vein Thrombosis (DVT): The calf location of the symptoms, combined with the patient's recent travel (which may have involved prolonged periods of immobility), raises the possibility of DVT, especially if the patient reports pain or swelling in the calf.

Leukocytoclastic Vasculitis: This condition involves inflammation of the small blood vessels and can present with a rash. However, it is less common and typically associated with more systemic symptoms or underlying conditions.
Erythema Nodosum: Characterized by the sudden onset of tender, red nodules, typically on the shins. It can be associated with various conditions, including infections, medications, and systemic diseases, but is less likely given the patient's presentation and lack of systemic symptoms.