Havent had my period in almost 3 years. I just started again and they last a little less than a month. Why? I am 24 years old.

Consider going in . For a workup? You should. You likely do not ovulate regularly creating proliferation of the uterine lining without an orderly withdrawel bleed. That is likely why when they do come it lasts a month. You may benefit from the pill but u need to get seen. No period in 3 years? You gotta go in for that.