I think you should see a doctor.
Is the blood bright red? That usually signifies hemorrhoids, but rectal bleeding that recurs should always be checked out. Hemorrhoids can be very itchy.
It could be herpes. Even if you are very monogamous, either one of you can enter the relationship with a pre-existing herpes infection and not know it. Also, if your partner has oral hsv1, and gives you oral sex, you can get genital hsv1. Most people with oral hsv1 never have symptoms.
You should check your screenings to see if they included herpes. My guess is they don't if you did them at your doctor. Another issue is that the hsv1 IgG test misses a full 30% of infections, so even if your partner has it, the test may miss it.
So get checked out. If they even think it might be herpes, ask them to do a type specific culture on it.
Good luck!