Thanks for posting your query.
I can understand your concern. Hydroceles are mainly symptomless and harmless. However large hydroceles cause discomfort because of their size. But still it is very unlikely for the hydroceles to cause pain after ejaculation. For the same you need to get sexually transmitted diseases like Chlamydia and trichomoniasis and prostatitis ruled out.
Also causes of testicular swelling other than hydrocele like a benign or malignant growth of testicles needs to be evaluated.
Local examination, urethral smear, urine examination, blood tests, prostate specific antigen, ultrasound scrotum and examination of the discharge from penis needs to be done to confirm the diagnosis.
Hope that this information helps and hope that you will get better soon.
Wishing you good health.