How To Cure Irregularity In Periods?

hi i am 20 years old and i got my period this month and i normally get it for 5-7 days but this month i only got it for 2 and a half days and then it stopped.a day later i woke up and i had blood in my under wear so i put a tampon on but when i went to change 4 hours later there was only a streak of blood on it. now i notice that when ever i wipe myself i see a faint streak of blood on the toilet paper. i have also been having pains on the left and upper part of my stomach. what could it be?