Period a week early

Jacky • 24

hi! What can cause a period to come a week early? I’m not supposed to get my period for another 8 days and I noticed some light bleeding when I wiped today and I’m experiencing cramps so I know for sure it’s my period wanting to start. but I just wanted to know what caused it? I’m not on birth control nor am I preventing pregnancy. & there hasn’t been any changes to my routine, I’m a sahm.