I got stung by a mosquito a week ago on my arm. Two days ago, it began swelling And is very red and painful.?

Likely infected. Thank you for your question! Mosquito bites can be very itchy. This is due to the body's reaction to the bite. Frequent scratching may cause mild infections that can resolve on their own. However, in your case, it sounds pretty severe. You should see a doctor ASAP! The doctor will likely prescribe an antibiotic to assist with healing, in addition to recommending OTC anti-itch medications.