Hello,
It can be due to an insect bite, contact dermatitis or an allergic reaction. You can apply an anti-itch cream or lotion to the affected area. Oral antihistamine, such as Benadryl can also be used. You should take a comfortably cool bath and wear smooth-textured cotton clothing. You should choose mild soaps without dyes or perfumes and use a mild, unscented laundry detergent when washing clothes, towels and bedding.
Please take a second opinion from a dermatologist if the symptoms persist. You may need topical and oral steroids.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I am going to delete this off my watch list, so if you still would like to talk, please message me.
There are numerous medications you can get to treat the affected area. What is not good, is that it is growing. If you do not know what bit/stung you, and the area continues to grow, it's time to see a doctor.
You can get bug bite/sting medications at the local pharmacy and just let them know what you need it for and the pharmacist can recommend the best thing for it.
Don't let it go to far though. Things to check for is growth both in area and in size of swelling, if the skin breaks, discoloration, and puss.