I'm 11 days late, and spotted for only 15 min, & i also took a pregnancy yesterday & it came out negative? why haven't i got my period yet?

Recheck in a week: You may just be having some fluctuation of your cycle. This can happen at times without obvious cause. Stress, thyroid and other endocrine problems can also cause changes. If it is isolated no need to pursue but if persistent irregularities you should investigate. Home tests are very good but some times just do not pick up early on. Do next test in am with first urine as will be more concentrated

Don't 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.

Irregular cycles: Unless you are having unprotected sexual intercourse, this is a change in your normal menstrual cycle, which requires further GYN evaluation and possible pregnancy test.