I'm getting a faint positive pregnancy test in the evening but when I do it in the morning I get a negative. Why is this?

That can happen. That can happen if the urine in the evening is more concentrated than the urine in the morning, or if a woman is pregnant and her HCG level rose enough during the day to make the evening test result positive. Sometimes, it happens randomly, if one of the tests did not work properly. If worried about pregnancy, one can repeat the test in 1-2 weeks. If pregnant, it should be very positive by then.