R.H.
Well, no similar situation to yours, but I think day 20ish would be a little late for implantation in a shorter cycle like yours; though of course, it is *quite possible* and not that uncommon I'm sure. The timing of your "mishap" and your spotting is spot-on however, for implantation and pregnancy. :-P
If I were you, I'd head over to the Dollar Tree and ask for some pregnancy tests (if you don't see them on the shelves). The ones I've had from there have always been the least expensive and the most sensitive (go figure!). Wait and use your 'first morning's urine' and keep checking until/if you get your period or a positive. If neither after another two weeks or so, you might wanna pay a visit to your gyno.
HTH & best wishes!