My cousin has a 2.5 yr old son. she's on birth control. having issues with her cycle & trying to figure out what's going on. according to her period tracker she's 4 days late. she's nauseous, breasts hurt and she got negative test so far. do another?

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.