Spotting brown since middle of last cycle. (today makes 3 weeks). Regular 28-34 day cycle. Am now 2 days late if it was 34 day cycle. Neg pg test.?

Probably. Irregular bleeding and not ovulating. To be on safe side would repeat pregnancy test but doubt highly you are pregnant.
Reasons: Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems. #1 reason is pregnancy.