After ejaculating, had achiness and dull pain in testicle. did not ejaculate for 2 months prior to this. referred to a&e, ultrasound clear, urine test done. was found with fluid build and and infection. given antibiotics. why did this happen?

I'm unsure whether you had a clean catch urine culture&colony count which documented a urinary bladder infection. Urine cultures don't detect it, but you may have epididymitis which causes testicular discomfort and is treated with antibiotics. Your symptoms were not characteristic of a prostate infection.

These are non specific symptoms and can be caused by many things- epidid. cyst, varicocele small Hydrocele (fluid around the testicle). Antibiotic do not help in this. Best is to get examined by a Urologist and wear a good support under wear in the meantime. Good luck etc.