Thanks for your query.Based on the facts that you have posted it appears that you have undergone vasectomy
7 months back and now have pain in left testicle due to engorged vein.
This is most likely to be due to Varicocele
which is common on Left side .However the diagnosis can be confirmed only on clinical examination and colour Doppler Study of the Scrotum
Please consult qualified Urologist
for clinical assessment and further treatment.
Do not worry this can be cured with pain killers or surgery (Varicocelectomy)