What if i accidentally left a tampon in at the very end of my period for almost 12 hours (i fell asleep) and now i have throbbing cramps?

Tampons: Retained tampons can cause infection and lead to pain and possible toxic shock syndrome (a blood infection causing shock and death). While this is unlikely after 12 hours of a retained tampon, you should monitor yourself for fever and foul vaginal discharge. You should check yourself by inserting a finger to remove the tampon if the string is not visible.