I will try my best to answer your query.
is one of the possibility. Periods may be delayed due to stress
is antibiotic, given to treat infection it has nothing to do with pregnancy or post coital contraception
I advice you to wait for week , then go for urine pregnancy test, using the kit which is available in any drug store, it is good if you use early morning sample of urine for test.
After getting the test result you can plan accordingly.
Hope my answer will be helpful for you.