I would contact your GYN. I, too, had a hysterectomy in Nov. 09 and have had no issues. The first few times I had sex there was a tint of blood but nothing major. The doctor probably placed a vaginal cuff at the top of your vagina. I would contact them and see what they say. I hope all is well!
This could be a blood string due to clotted blood. It is normally seen in heavy bleeding. So you should get yourself examined and treated accordingly.