how long

so i took a pregnancy test yesterday since my period is now 4 days late — it came back negative but i have yet to have my period. i was on prednisone and an antibiotic for being sick earlier this week. i’ve also been under a lot of stress since my period hasn’t happened yet. when is it an appropriate time to go to the doctor and ask for help? or should i wait it out a few more days for my period to happen?