I expected to start my period on the 18th but i started bleeding today. it's lighter than usual. could i still be pregnant? we're trying to conceive.

Maybe not, but...: A urine pregnancy test, done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a 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 gets her normal regular period, then she is not pregnant, and a test is not needed.