I am 6 days late for my period, but i have taken two tests three days apart and they both have said negative. what could be the cause for this?

Some women who are usually very predictable can skip a cycle occasionally. The amount of delay is unpredictable and there are no real symptoms other than the delay & negative pregnancy tests. If it goes past two months with a negative pregnancy test many OBGYNs would offer meds to bring on a menses. By that point it would usually be quite heavy.