Hi and welcome
I'm sorry for what you are going through. The enlargement of your kidney sounds like hydronephrosis. Have they done an ultrasound recently or any other scans?
Did they tell you how well this kidney is functioning at? There's a test called lasix renal scan, I'm unsure if they do this on people with one kidney. There are other tests to perform though. A 24 hour urine collection with blood work is another option.
As for the cramping did the doctors ever mention renal colic to you? This usually happens to people who get or have kidney stones, but it is known to occur without stones.
I know you are 24 another possibility is your prostate possibly being the cause of having to go to the bathroom so much. I'm not trying to blame it on this but it's a slight chance it may be this.
I would talk to your urologist about your concerns, I know you said you haven't been in a while. With the scarring in the kidney it's important to have that followed up with.
Please keep us posted.
All the best..