Well it can be a number of causes, assuming you are a healthy young individual and there is no irregularities of the testicular tissue to the touch, it could be just a simple bacterial infection of the tubes that transfer sperm, that is progressing towards the prostate. However this is no diagnosis and what you have stated is not enough to make one, go to a doctor, as things that are diagnosed early are treated easily and efficiently.
This sounds a lot like prostatitis to me - uncomfortable sitting, discomfort in the testicular area & when ejaculating. Could also possibly be epididimytis, some of the symptoms are similar. I think this is what Klamidia was talking about. Your reg Dr should be able to diagnose this - prbly will check the prostate & may ask for a urine sample. If you don’t get relief after seeing the Dr, you also could see a Urologist, altho I’m pretty sure your reg MD will get it right - what you’re describing is pretty common, I’ve had both in the past myself...