Even though this is not your first pregnancy, please remember that no two pregnancies are alike.
At 20 weeks, you should have been to the doctor, received a good checkup, and gotten more information than what you have about the progress.
Please see your doctor, and as suggested, get an ultrasound.
Good luck, and please keep us posted.

We can help reassure you in most cases, but we cannot examine and/or diagnose you through the internet. Nothing will, or should take the place of a doctor you trust.