How do I go about requesting an ultrasound?

Patient: I am reaching my fifth month without a period. I have seen doctors that have done urine pregnancy tests but refuse to do a blood test or ultrasound because of the negative urine tests. My stomach has begun to swell around my belly button and my breasts have grown a cup size in the last four months. I am twenty five years old, married and I would really love to have a baby. How do I request an ultrasound from my doctor that doesn’t want to provide me with that service? Even if I am not pregnant, I would like an ultrasound just to ease my mind.

Symptoms: Swelling stomach with no weight gain, fatigue, irritability, minimal pain (like period cramps with no blood at all)