I am currently experiencing a very light but very painful period. it was 4-5 days late. i am sexually active. should i be concerned?

Hard to tell: The fact you got it is good in that it probably means you are not pregnant. Although, if it was unusual, you may elect to have a pregnancy test done just to be safe. If it was the first of its kind, i would not worry (provided you are not pregnant) and just wait for the next one. Chances are, it will be a normal one.

Take a HPT: Since this is not characteristic of your normal period - would go ahead & take pregnancy test. Reasons for missed /late periods: low body weight, obesity, marked weight gain or loss, over-exercise such as in endurance athletes, breast-feeding, stress, illness, eating disorders (bulimia or anorexia), travel, hormone irregularities, drug use, medications (i.e., birth control) or med problem (PCOS)