2 days late period and very light, lasted only day. could this be implantation bleeding?

Depends: It is possible that this may be implantation bleeding if ovulation/conception occurred later in one's cycle than expected. Because if truly regular cycling periods, then time of expected menses is past the timing of implantation.

Pregnancy is: possible - take HPT to know. 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 (i.e. PCOS). #1 reason is pregnancy.