Nurse thought I was farther than 7 weeks?

I recently went to the health department for some tests and I had told the nurse I was pregnant. She asked how far along I was and I said about 7 weeks, I haven't been to the doctor yet but I know this for a fact because it only happened once. When the nurse was pressing on my stomach she asked if there was anyway I could be farther and I told her definitely not. I've already had a son, who is now 9 months so maybe my uterus is growing faster and that's why she thought I was farther. It just made me nervous cause now I'm worried about the possibility of twins? I've had two dreams about it prior to this and I'm just curious. Twins don't run in my family, but they don't have to to occur.