Hi, Doctor. I Am Sharmi, 26yrs Old. I Have Irregular

hi, doctor. i am sharmi, 26yrs old. i have irregular period cycle for 3 months (july 25th, aug 1st, sep 8th) my last period was 8th sep and i had protected sex on 7th october wid my bf. but still i dnt have my period. now 21 days passed from expected date. i took urine hcg tests on 18th , 20th oct, both r negative. if any possibilities there to be a pregnant? i am so stressed and last 18th had a back pain like mentrual pain. plese clarify me doctor