What Causes Irregular Periods?

Hey there, I'm 18 years old and I got my period last month twice and I haven't got it this month and it's almost over. My first period last month was on the 7th and the second one was on 20th, both were 5 day regular periods. And now it's the next month and I haven't gotten my period, I have been sexually active and haven't used protection either. I took two home pregnancy tests and both came out negative. I have no clue what's going on with me.. Help please!