Hello,
It can be due to an insect bite. However it needs to be evaluated right away and testicular torsion ruled out. Testicular torsion is characterized by excruciating one-sided testicular pain, with sudden swelling and is a medical emergency. You need to get a testicular ultrasound done to completely rule it out.
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.
All other stds negative at 5 weeks blood and urine