The discharge can be due to an infection as said above, especially since you describe it as having a fishy odor and say it's yellow. Period blood smells different...not fishy.
It is possible you could be pregnant if you did not take the morning after pill correctly (within 72 hours after unprotected sex, sooner the better). However, the morning after pill is a high dose of hormones and has been known to delay periods in some women. You could take a pregnancy test in a few days (too soon currently) to determine what's going on. If you still don't have a period even after a negative test, you could test again in another week or so or visit your doctor for a much more sensitive blood pregnancy test.
If periods do not resume in a few days it will become necessary to take a pregnancy test. The yellow discharge can be due to infection or due to mixing of blood. Keeping the region clean and dry may help. If not consult your Doctor.
Hope this helps.