What Causes Light Vaginal Bleeding And Nausea While UPT Is Negative?

i havent started my menstrual period in about two months and in one of those months i went to use the bathroom and when i cleened there was a little blood so i thought i was my period but it wasnt normal like i usualy have it so the next day its gone and i took a home pregnancy test and came out negative and i have headaches and naucia but not to the point were i throw up. I have a three year old child and ever since he was born i havent had any birth control and my periods have always been irregular. what can this mean.