First of all, it is always unreliable and possibly unfounded advice that anyone can both give and receive from medical queries on the Internet. For proper medical advice based on your own personal health records and especially if you are in any pain, you should go to see a doctor as soon as possible. However, possible causes could include:
- This could be a bite from a spider that has made the initial bite and then walked around biting in other areas. It is possible this could have happened while you were asleep.
- It may not be a bite but an allergic reaction to something. If it increases in size or gets very painful, seek medical advice as soon as possible.
- Tick bites can leave circular bites that look like a bull's eye. They usually do not cause any pain or discomfort. If you start experiencing any flu-like symptoms such as joint pain and feel weak or tired, go to a doctor as you may have contracted Lyme disease.
- It may be a mosquito bite if it is very itchy. Try to avoid scratching it and put some calamine lotion on it. If it does not go away after a few days or if the symptoms do no alleviate, go to a doctor.