*hugs*
It sucks when you can't be sure.
On one hand, I'd say your internal 'clock' as it were might be out of sync from the graveyard / shift work etc, and this could definitely affect your cycle, as well as any other stresses.
On the other hand, it could have been a chemical pregnancy but these things are hard to establish - are you TTC again yet?
I hope you are able to find some answers, or at least some peace within yourself about it.
Edited to add: As you probably know clots / spotting / general cycle wackiness are run of the mill for me, so I don't mean to come across as blase about it! Just wanted to be clear
Edited by Pearls