While many of my answers on AB are based upon research, rather than knowledge or experience, this is one time that it's definitely a case of experience for me.
Nearly 50 years ago (when I was still a student) I woke up on Boxing Day with the same symptoms that you're describing. (i.e. the smaller of my two testicles had suddenly become far, far bigger than the other one and it hurt like hell!).
I later apologised to a doctor for calling him out on Boxing Day but his response was "Don't worry about it. I've had what you've got and I know how much it hurts. I'd be seeking help as soon as possible if I was in your position too".
The condition is called orchitis, which you can learn more about here:
https://www.mayoclinic.org/diseases-conditions/orchitis/symptoms-causes/syc-20375860
The doctor provided me with antibiotics (straight from his bag, with no need to visit a pharmacy) and pain-killing tablets (also straight from his bag). Those tablets were so strong that he insisted upon calling my mother in as a witness to the fact that he'd warned me that exceeding the stated dose would lead to certain death. I don't know what they were (although my guess is that they were some form of morphine) but taking just a single tablet quickly numbed the whole of my body in the same way that a dental anaesthetic numbs a gum. (i.e. you could have stuck a pin in any part of my body and I wouldn't have felt it).
I felt a lot better as soon as the painkiller started working and I was fully fit again within a couple of days.
Get some treatment for it now!