When did i most likely conceive?

Just curious. I worked hard for a year trying to conceive with no luck. Charting ...the whole nine yards. It finally happened after giving up and relaxing about it. Just for giggles... when do you think it most likely happened. My last period was sept 18. Had on Sept 22, Oct 1, Oct 2, Oct 3, Oct 6. I have a 27 to 28 day cycle. I should have started on Thursday Oct 15. Period never showed. had implantation spotting late Saturday evening the 17th. It was light pink only once when i wiped. I tested negative yesterday and finally got the BFP today!!!! I think working so hard and stressing about it made it more like a job and took the fun out of BD with the hubby and the stress probably made it more difficult to happen. Hang in there ladies and try not to stress!! Good luck to you all. I always thought implanation spotting happened before missed period. Does that mean i probably ovulated at a later date?