18 days late for period, no positive pregnancy test from home, why is this? and what should i do?

Lots of reasons: There are lots of reasons why your period may be late. Are you overweight? If so, you may have stopped ovulating. If you are not ovulating, you may have irregular bleeding over time. Pregnancy tests can also remain negative, despite being pregnant, for quite some time. If you don't have some bleeding within the next couple weeks, see your doctor. Have them repeat a pregnancy test and try OCP

Could be: Could be anovulatory cycle or irregular ovulation, highly unlikely for pregnancy if the test was negative. This is not normal but it happens often and is generally not to worry. You can see a doctor anytime but it's also okay to wait up to 1 additional cycle to see if the period comes. good wishes.