Cellulitis: This is a common bacterial skin infection that can occur after an insect bite, especially if the bite becomes scratched or infected. The symptoms of cellulitis, such as redness, swelling, warmth, and pain, match the description of an insect bite getting worse and painful.

Allergic Reaction: Some individuals may have an allergic reaction to certain insect bites, which can cause increased swelling, redness, and pain at the site of the bite.
Infected Insect Bite: Similar to cellulitis, an infected insect bite can become worse and painful due to bacterial infection, often requiring antibiotic treatment.
Folliculitis: This is an inflammation of the hair follicles, which can occur after an insect bite, especially if the bite causes scratching that leads to follicle irritation.

Necrotizing Fasciitis: Although rare, this is a serious bacterial infection that can start with a minor injury like an insect bite. It's characterized by severe pain, swelling, and redness, and can lead to serious complications if not promptly treated.
Erythema Migrans (Lyme Disease): If the insect bite is from a tick, there's a possibility of Lyme disease, which can present with a characteristic "bull's-eye" rash and flu-like symptoms.
Sepsis: In severe cases, an infected insect bite can lead to sepsis, a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs.

Methicillin-Resistant Staphylococcus aureus (MRSA) Infection: While not the first consideration, MRSA can infect insect bites, leading to more severe and resistant infections.
Leishmaniasis: In certain parts of the world, sandfly bites can transmit Leishmaniasis, a disease caused by a parasite that can lead to skin lesions and systemic illness.
Kulzori (Sandfly Fever): Another disease transmitted by sandflies, it causes a self-limiting febrile illness but can be considered in the differential in endemic areas.