I came across this thread because I have similar symptoms, and in my case in the past it has been due to internal hemorrhoids (sorta embarrassing but you seem like nice people!) -- meaning it's inside the colon, I suppose you'd say, and a blood vessel gets swollen and/or bursts in a painful, cramping fashion. This can be brought on by certain sex positions (especially if your partner is, ahem, well endowed), and it can happen in the middle of sex, i.e. -- Ow ow, cramp, stop! And then if you are unlucky you will get some blood on the TP the next time you use the bathroom, but not always. This might be to blame! Just an idea.