It's not unusual for someone who is carrying a load of anxiety, stress or guilt over a sexual event to transmit that into fear of a less likely worst-case scenario that could come from such an event.
In the forums where people write in asking about sexually transmitted diseases, lots of people write who have transferred guilt over cheating into unrealistic fear of having an STD. (Even if all tests show they don't have one, they still are afraid they have one.) This is because for some people, it's easier to stress over getting an STD than it is to face up to the fact that they had sex with the wrong person.
In this community, a lot of women write in who have let their stress over a sexual event settle on the question of who the baby's father is, even if they (deep down in their heart) can see that the test was correct, or the dates show the wrong person is not the dad.
Reassurance about the question the person is asking ("Do I have an STD?" "Is the paternity test right?") doesn't reassure, because the thing the person is asking for reassurance about is not in fact the thing that is actually bothering them.
It sounds like you are looking in the face of medical evidence of two kinds, a DNA test and an ultrasound, and choosing instead to believe (or half-believe) stuff you have read on the Internet. The Internet is full of things that come from sources with their own hidden agendas. It is not medically based. It is not the truth of the Lord. It is not familiar with your medical record and it has not taken a DNA test for you. Please understand that you are doing this to yourself, to justify your unsettled reaction to the rape.
What are you afraid will happen if you tell your boyfriend you were raped? Do you think he will leave you? Why didn't you report the assault and press charges? If the police still have the record that the date-rape drug was found in your system, it's not too late. Are you afraid he will claim he didn't put it in your drink? Even pressing charges will cause there to be a record that some other woman can find some day when she is deciding whether to marry this guy. And all of this will allow you to take back your power. Rape is not your fault. It is a crime and a crime was committed against you. Turning your back on this assault is turning away from your own autonomy.
If you think the costs will be too high, to yourself, of reporting this man and making him accountable to the authorities for his crime, at the very least, see a counselor and get the burden of silence unloaded. You're carrying more than you should carry.