21 Days Late

Hi girls I have a problem I took 7 pt the first two came out positive that same night I freaked out and bought three different kinds and they came out negative. Two weeks ago I went to the dr to confirm and the test came out negative. Now again I went back and it was negative again so dr did blood work. I won't get the results til Friday but I still don't understand why I'm 21 days late on my period and why the first two came out positive... Anyone ever have these problems before ??