What Causes Missing Periods And Spotting?

I'm 18 years old, about 5'1" and my weight is 190......My period is two weeks late. the last time i had sex with my boyfriend was a month ago (he used a condom). On April 24 till now April 27, i had very light bleeding, its like pinkish/orange sometime brown and light red. I did a HPT it came out negative, what could this be? i did research and it could possibly be "ovulation bleeding"? i have never had this before with my period, and i do have irregular periods.....PLEASE HELP