For the last 2 days i've had light vaginal bleeding and cramps. today, i started having lower back pain as well. what could it be?

Anything: Without knowing someone's complete medical history and menstrual cycle pattern previously it is hard to say what the cause of the symptoms you are having are due to. Ovarian cysts, pregnancy, miscarriage, tubal pregnancy, infections , etc. You should be seen by a physician for an evaluation.

Need to examine: It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. If you had sex since your last period and missed your period by 4-5 days, do a home pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully.