Ay sis, ganyan din nangyari sa akin dati after we did the deed around ovulation time, ang cycle ko tumalon ng dates kaya grabe ang confusion! Based sa what you shared, possible na nag-shift ang cycle mo, but honestly kailangan mo talaga magconsult with your OB para ma-clarify kung anong nangyari, baka implantation bleeding yung light period mo on Oct 16-17, or baka nag-adjust lang talaga ang body mo, kaya better safe than sorry na mag-check. For now, try counting from the first day ng heaviest flow mo (so either the Sept 27 or Oct 16 date), pero better talaga if you track with your doctor para clear ang picture moving forward, lalo na kung palaging regular ka before. Sending you love, mama!