Could I Still Be Pregnant Despite Missed Period, And Lower Back Pain, Given My Recent C-sections And Use Of Calendar And Withdrawal Methods?

Good day, doc. I had my first child via Cs about 3years ago. I had myomectomy 1year 6months after his delivery. A month after I got pregnant and delivered through cs again. The baby is now 8months and I missed my period again. Just did a pregnancy test and it s negative. I have being having lower back for more than a week now. Please could it still be pregnancy because I have being using the calender and withdrawal methods of contraception. Thank you