Should I get a 2nd ultrasound?!?

Ok, according to my charts February 12th was the first day of my last period, but then I was looking in a journal and I had said that Feb. 12th was the LAST day of my period....I started having visual changes on Feb. 22nd, and went for an eye exam, shortly after that I found that I was pregnant. I am fairly positive I was at LEAST a month along when I went to my eye appointment, my Dec.-Jan. periods were really funky and not normal, so it is possible that I am further along than anyone thought....the doctor said that according to the first ultrasound I would be due on November 27th, but I don't agree because my son measured small at first, but then I had another sonogram at 5 1/2 mo. and he was like 3 weeks AHEAD and so they moved my date 3 weeks ahead.....should I get a second opinion, because it has been about a month since my last one, giving the baby time to catch up if it is going to, my midwife is going to be gone in early Nov. so I want to KNOW if she/he is due during the time she is gone! (she does have a back-up but I don't know if I like the back up)