What could it be? What can cause this allergy?

You must see a dermatologist. This could be lyme disease, ringworm as well as several other skin conditions. I've seen this circular rash before on blogging sites others have sent in, it's not uncommon.
It's good you are searching now because you must self educate yourself before you see a dermatologist. Why? Dermatologists make mistakes so learn all you can about circular rashes.
A good site that helped the diagnoisis of my Perioral Eczema, which is a circular rash around the mouth, is dermnetnz.org. This skin site covers the most skin conditions of any site I've found and it covers rare ones others don't cover and best of all offers a good selection of photos of rashes. I kept searching until I found a rash that looked like mine.
It's very important you educate yourself and start a journal and list all possibilities. Here's a personal example. I had been seeing 2 different dermatologists seeking treatment for my rash around my mouth. They had been treating this rash with steroids believing it to be caused by my Allergic Contact Dermatitis issues, yet my rash went on for months not improving. I learned on dermnetnz.org that steroids fuel PE. The dermatologist's got it all wrong. I stopped the steroids. After seeing these photos on dermnetnz.org I went in and told them I believed it was PE they simply agreed with me and agreed I should stop the steroids. I believe now it all started because I was using a nasal steroid spray. Of course I stopped that also. It still took several months fade out and stop itching. If I had self educated myself first I could have put off months of intense itching and embaressing rash with no way to hide it.

Hello @annagavi This may be time to visit a doctor or dermatologist to get an answer. You’ve had the rashes for 3 months on different areas of your body and they seem to be enlarging. It’s probably a good idea to have a medical professional check this out.

The lesions look similar to Ringworm…which can start out as small rings and grow. Ringworm isn’t actually caused by a worm or parasite, but a fungal infection. However it’s usually very itchy and scaly looking.
Do these areas of rash itch? Have you tried any over the counter treatments such as Cortisone Cream or antifungal?

Ringworm was my first thought as well.