I am on oral contraceptives and usually have 3-4 day periods. this last one came on time, but only lasted one day. could i be pregnant?

Unlikely, but...: A urine pregnancy test gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a "withdrawal period". If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and thus lead to a repeat test. If a woman keeps getting her "withdrawal periods" while on the pill, then she is not pregnant.