Thanks for your query.Based on the facts that you have posted it appears that you have
3 weeks back ans have noticed a small lump in your scrotal sac.
Most likely this is due to either Epididymitis
around the suture material that has been used for ligation of the Vas.
However this can be confirmed only on clinical examination.
Please consult the Dr who has performed the surgery .
Please take antibiotic and anti inflammatory medicine like Diclofenac
twice daily.It will get resolved in 2 weeks.