I had my period for 1 day last week and now im having really bad cramps and lower back pain my period usually last a week, any idea what this could be?

Short periods : Short periods are very common if you are on birth control pills. Sometimes could be associated to ovulatory dysfunction, early pregnancy, or hormonal problems. I recommend to run a home pregnancy test and make an appt with your gyn doctor if the test is neg and short periods persists.

Must R/O pregnancy: First, could be an ovarian cyst , endometriosis , an infection, hormone problems or other things so you need to see a gyn for an exam and toatl evaluation.