Implantation Bleeding or Constipation Bleeding?

How do you differentiate implantation bleeding from blood resulting from abrasions due to constipation? 
To clarify: You can't get pregnant if your partner ejaculates on your thigh and you're wearing a panty with a sanitary napkin, right (note: my period was already over at this point)? Is there even a possibility for the sperm to reach my vulva/vagina?