Better to do the pregnancy test about 1 or 2 weeks after the expected period to come. Which would be around the 5th or 6th week of pregnancy.

Do it in the morning like the others mention.

If still negative, then period might be delayed due to stress or other things.
If positive then congrats! Find a good gynae and prepare to be a father!

p/s: Yes, certain medications might affect our body hormones, which would cause delayed/advanced period sometimes.