Thanks for your query.Based on the facts that you have posted it appears that you have noticed change in colour of your semen which is yellow since very recently.
Most likely this is due to infection either in Seminal Vesicle
Please consult qualified Urologist
for clinical examination You need to undergo some test
1) Routine urine test
2) Semen examination a
scanning of the abdomen.to establish the diagnosis,Further treatment.will be
decided by him and will depend on results of the tests.