Period is 28 days late. 4 negative urine pregnancy tests 1 negative blood test. Starting to get cramps/sharp pains what could this be?

Period starting? Sometimes when you miss a period, the next one will come about a month later at its "usual" time. So, maybe the cramping is happening because you're about to get your period. If you don't get it in a few days and the pain persists, see doctor.