What a stressful, difficult time you’re having. I’m so sorry. Love should be sweet and comforting and fun, not like this. It sounds to me like your girlfriend has very conflicted feelings about being sexual at all. Her fears of pregnancy may well be grounded in guilt that she is even exploring sex with you. She wants to be close. She has normal feelings. But then the guilt sets in and she gets so anxious she can’t think straight.
Your girlfriend doesn’t need any more ultrasounds. She does need to see a counselor. Her anxiety is depriving her of enjoying her relationship with you and being comfortable with her own sexuality. If she won’t go to a counselor alone, go as a couple. With a counselor’s help, you can learn some new ways to comfort her and to help her be more relaxed about sex.
Meanwhile, take a big step back in how intimate you are with her. She is too fearful to be nude or to participate in mutual sexual exploration. Stick with kissing and hugging until she gets the help she needs. As frustrating as that may be, it’s better than what you’ve been going through.
I wish you well.