Insect Bite Reaction: This is the most likely diagnosis given the history of insect bites and the description of the rash as maculopapular and petechial, which is consistent with an allergic reaction to insect bites. The itchy nature and the spread of the rash upward from the legs also fit this diagnosis.

Contact Dermatitis: This could be considered if the patient has come into contact with an allergen or irritant, aside from insect bites, that has caused a skin reaction. The rash's appearance and the patient's symptoms could align with contact dermatitis.
Urticaria: Although urticaria typically presents with wheals, in some cases, especially with chronic urticaria, the lesions can be more maculopapular. The itchy nature of the rash could suggest urticaria, especially if the patient has a history of allergies or autoimmune conditions.
Erythema Multiforme: This condition can present with target lesions and can be triggered by infections or medications. While less common, the petechial aspect of the rash and its spread could potentially fit this diagnosis, especially if there's an underlying cause such as a viral infection.

Meningococcemia: Although rare, this condition can present with a petechial rash and is life-threatening. It's crucial to consider this diagnosis, especially if the patient has symptoms such as fever, headache, or stiff neck, and to promptly treat with antibiotics if suspected.
Rocky Mountain Spotted Fever (RMSF): This tick-borne illness can cause a petechial rash, fever, and headache. Given the history of insect bites, it's essential to consider RMSF, especially if the patient has been in an area where the disease is common.
Thrombocytopenia: A low platelet count can cause petechiae. While the itchy and spreading nature of the rash might not fully align, thrombocytopenia is a critical condition that could explain the petechial aspect of the rash and should not be missed.

Leptospirosis: This bacterial infection can cause a rash among other symptoms like fever and headache. It's typically associated with exposure to contaminated water.
Vasculitis: Certain types of vasculitis, like Henoch-Schönlein purpura, can present with a petechial rash. These conditions often have additional symptoms such as joint pain, abdominal pain, or kidney involvement.
Drug Reaction: Certain medications can cause a rash as part of a drug reaction. While the history provided focuses on insect bites, it's essential to consider any new medications the patient might have started.