How To Treat Cyst In One Ovary?

im 16 yrs old & im sexually active, i usually have a regular period & mine is now three weeks late. I went to the dr. after it being 1 week late and the blood & urine pregnancy test was negative. I thought it could've been the small cyst i have on one ovary, but they told me that it wouldnt cause it to be that late. I then thought it could've been because i have a SLIGHTLY elevated thyroid but my mom(whos a nurse) told me she didnt know. Do you have any idea what it could be? or is there a chance i could be pregnant even though it was negative after one week of being late?