I think that stomach pains are just a really common early pregnancy symptom, common in normal pregnancies. Mostly they are due to the hormonal changes causing IBS like symptoms, or due to the uterus beginning to expand a bit.
I really doubt you can "feel" the placenta implanting. I don't think it implants before about 10 weeks either.
I know what it feels like to be driving yourself mad trying to work out what went wrong, but please try not to; there was nothing you could have done and no way at all that you (or anyone else) could have known what was going to happen. You were just incredibly unlucky (& Ricky too). It gets to me that my experience was so so far worse than normal; it was even much worse than "normal" PE. But it just happens to a few women and for no obvious reason we were among them x