positive then a negative

I got a positive and then a few days later a negative. I am now 21 days late my boobs hurt, I have been getting sick at the sight of foods. I know the mind can play tricks on the body. however, I do know people who have had similar expereices and still have been Pregnant it just wasn't showing on the hpt even months into their pregnancies . I plan on going to the Dr after I have missed a full cycle at 28 days. Please let me know of you have any similar stories. Please keep comments positive I do know all the reasons I could have had a false positive or could not be pregnant. Thank you!