Ultrasound for a big baby?

We are going into the 26th week and another ultrasound is being ordered. On Monday, I saw Dr. Malak, my regular Ob/Gyn doctor and he said I was measuring big. He ordered an ultrasound for tomorrow (Monday, July 20th) to make sure everything looks okay. He said there may be extra fluid, or that the baby may be growing too fast. My sister said the same thing happened to her and she had a big baby. Ainsley was only 6lbs 2oz, which is not considered big. I guess I've been a little worried about it. Worried about what it means if the baby is growing too fast. Would that be a form of encephalitis? I'm not overly worried because I promised myself that is something I will not do during this pregnancy over things that I cannot control. So, I'm just waiting patiently for Monday to come- and today is Sunday - and I've been filling out this sheet all week recording the baby's movements in a given hour every day. She's a mover and a shaker!