There's no way of knowing what you're facing here because it may not be anything, or it may be serious.
Do you feel any lumps?
This has a good description on how to do that - https://ro.co/health-guide/check-for-testicular-cancer/
You really need to see a doctor, even if you don't feel lumps. It may just be the way you are sitting, but it could be something serious, so don't delay it, and just go. Let us know.