What Causes Pain And Swelling In Lower Left Pelvis And Irregular Periods?

I have been have sharp lower left pelvic pains. they come out of no where. I am a 20 year old female i have only had one sex partner and as far as ii know i am his first and only. I have irregular periods and today i noticed that i have some swelling in my lower left pelvic area and it hirst more than nomal. also i did not have a period last month or so far this month.