Okay so my period just when off week before last , and its not supposed to come back on till late next week... but it came on this Tuesday January 14th , and went off today January 17th.. its super light , lighter than any period I've ever had , the blood was light red the first two days , and the last two it was brown yellow looking . All the blood from these 3 days wouldn't even fill up one whole pad... also I had sex on January 4th and my boyfriend said he did inside me.. any clues on what it might be ?

Could be an off cycle, but pregnancy is certainly going to be a possibility as well. Are your periods usually regular? See what happens when you would expect the next one and test if need be.