What Causes Abdomen Cramps With Periods For Only A Day?

Hi.im 17yrs old, 5ft 7 and weigh about 12st. Ive been having stomach cramps the last few months, ive also had what I bleieve was my period but it only lasted less than 24 hrs with a brown discharge kinda thing. Ive been feeling really run down lately and also noticed my stomach is a little hard right by my belly button. I took a home pregnancy test but come back negative. Ive also gained abit of weight lately but i dont eat alot. What could this mean? Thank you