Could just be that you're not quite 5 weeks along. Counting from the first day of your last period is not the most accurate way to know when you ovulated this time; if you ovulated a little late, you could still have an embryo too early to see.
Have you been scheduled for a follow-up ultrasound, or is your doctor continuing to do blood tests? If your hCG continues to rise as expected, which would be determined by a blood test, then that's a very good sign.
Sometimes it is too early. You may have your timing off just a tiny bit. When do you go back to the doctor? Does a home pregnancy test register positive? Let us know what is happening. hugs