I had a period delay pill and had protected sex butnow i got periods for a day only, usually i have 4 days long.what could be the possible reason?

Vaginal bleeding: It is difficult to answer your concerns because : your body may be undergoing some type of stress (cold virus, physical stress like not sleeping enough, emotional stress from daily living, etc...) which could change your cycle dates. It could be irregular vaginal bleeding or something more serious. Please have a pelvic exam & discussion with your GYN physician. Good luck