Kathy,
It is possible to get pregnant from sprem in pre-ejaculate. The symptoms you are describing sound like they could be
pregnancy symptoms, especially since you haven't had a period in three months. However home pregnancy tests are usually pretty accurate. If you had three negative tests maybe there is another explaination for the changes in your body. I don't think you are overreacting. Blood tests are more accurate than urine tests. I have heard of women who were pregnant but never got a postive reading with urine tests. The ultrasound might detect other problems, such as cysts or other problems. Several years ago I had an ultrasound done because I was having pain and the doctor wanted to find out more with what was going on in my reproductive and digestive systems. I wasn't pregnant, it turned out that I had irritable bowel syndrome actually. Best Wishes.
Kate