Is there a possibility to get false negative pregnancy result six weeks after sex?

Hello doctor,

I am a 20 year old female and had sex for the first time. I took Regestrone to delay my periods because I planned to get intimate the same day as my periods. I took them for three to four days before my expected period date and had the tablet two to three times a day. Last dose I had it just the night before having sex. I had sex many times the next night but with protection and took an i-pill to be sure. Next week got light periods that were very painful. But ignored it thinking it was because of hormonal imbalance. Both of us were affected by flu and it turned into a stomach infection with alternate periods of diarrhea and constipation that was accompanied with mild nausea but had no vomiting. I became very weak and then eventually recovered by taking Pantocid, and Digene. I am six days overdue and had slight back pain, nausea and increased urination. I do have migraine issues as well as I am quite obese with a BMI of 33.8. I got two home pregnancy tests done and both of them turned out to be negative. However, I still have not got my periods yet and I have been insisted to check for PCOS and get an ultrasound done. I am worried if the ultrasound shows that I am pregnant. I have gained about 4 kg. Is it possible to get a false negative six weeks after having sex? What can I do to get my periods early? Please advice.