Possible changed due date

So last friday we went in for our "original" 20 weeks ultrasound but we were actually 21 weeks. We cheated and went to fetal fotos at 17 to find out the gender which now, after this experience, I'm so glad! The ultrasound tech at my Dr.'s office acted like she just wanted to hurry and get this done so she could go home. She didn't even explain anything and I couldn't really see what she was doing because she was going so fast. I kept having to ask questions and I wasn't impressed with her at all. The last ultrasound tech we had was so personable and showed us detail by detail what she was looking at and took like 10 minutes just letting us see our baby. This girl just went through and did the medical scan through then said everything looks good except the baby's positioned on his stomach so she can't see his heart. The spine was reflecting too much so she said we'll have to come back. THEN.....she asked me how many weeks I was and I said 21 and she said that the baby was measuring 19 weeks so she's pretty sure they'll change my due date to like May 10th instead of April 25th! Not that big of a deal but to me, she just took 2 weeks away of my life! I was so bugged! haha I was most surprised because at all my Dr's visits he keeps saying I'm measuring right on and everything is right on track?? Has this happened to anyone else? I dont want them to push it back!