Based on good medical evidence, the doctor was saying your conception date was the 12th of January, which of course means nothing would have remained of your assailant's sperm in your body at that time and there is only one guy who could be the father, your fiance. Did you report the assault to the police? That is one way for a woman to take back her power after she finds herself in a situation in which she is powerless.
You didn't tell your fiance that you got assaulted. You were afraid you would be blamed (it's quite likely your assailant would have claimed it was consensual), you evidently were afraid your fiance would not believe you, and you basically kept the whole thing silent. That is a way to survive what happened in the immediate aftermath of the shock. But it doesn't work in the long term, once your emotional reactions get over the primary fear. What you need to do now is talk to a therapist or counselor. Women don't report rape for a lot of reasons, especially because they feel they won't be believed. But the unfairness of not being able to give vent to their anger at being forcibly victimized can keep them responding to the event for a long time in emotional ways that interfere with their life.
So, talk to a counselor. Describe what happened, how you felt then, and how you feel now. Your goal is to stop this crime from carrying forward into your life. The guy did something to you back then, don't let him do it to you every day from now on. That's how you take back your power.
Once you reach some kind of emotional closure about the assault, the logic of what the doctor and the ultrasounds are telling you will have a better chance of sinking in.
Also, read the other ladies' stories in this community -- you'll find that when you're reading someone else's story you can easily see who is the dad even if the poster is too worried to see it. Sometimes this helps a woman look at her own question with fresh eyes and see it's as clear as their stories are.