Missed periods. I see a gynaecologist?

I'm a 20-year-old woman with historically very regular menstrual cycles. My last normal period started on August 13, 2026. I had condom-protected intercourse on August 20, with no known condom breakage or slipping, and no intercourse afterward. On August 27, I experienced unexpected bleeding lasting approximately one day. My expected September period never arrived, and as of October 11, I still haven't menstruated. I took two home urine pregnancy tests approximately 37 and 45 days after intercourse. Both were negative, including the second test using first-morning urine. My questions are: 1. Can pregnancy from August 20 reasonably be ruled out based on these tests? 2. Is a quantitative serum beta-hCG test medically necessary at this point? 3. Since my cycles were previously regular, what investigations would you recommend for the missed period? 4. How soon should I see a gynaecologist? I've also been experiencing considerable stress recently. Thank you.