If you get a urine test two weeks after sex and a blood test the week after the urine and they are both negative can you trust the results?

Blood test, yes. If the last episode of sex was three weeks prior to the blood test and the test was negative, the result should be reliable. You may consult this site for early signs of pregnancy. http://www.webmd.com/baby/features/8-early-signs-of-pregnancy.