Measuring behind?

Ka

I'm 28 weeks and I see my Obgyn every 2 weeks. Today was my 2 week follow up. She measured my stomach and said I'm measuring a few weeks behind and that she would get me an order to get an ultrasound done. I'm really worried and wondering if anyone else has gone through or is going through the same situation. His heartbeat was strong and she said it sounded like he was, "excited." Because it was strong and rapid.