The weight loss is only something to be worried about if you don't have any weight to loose. If you're average, to overweight, and you are still healthy it's really nothing to worry about. I'm sure your doctor/midwife would say something to you if they were concerned, but if it is bothering you ask at your next appointment.

As for bubs measuring 7 days behind. What is really important to remember is that they are measuring from guesses of where the baby end (ITMS?). As in they can't get inside you to actually see how big bubs is so they are measuring off an ultrasound, which is basically a fuzzy picture. So they are guestimating where to put the points to measure from. Granted they are educated in how they do it, and they do it a lot, so their accuracy is pretty good, but at the end of the day it is still just an estimate, which is why they give up to a 2 week leeway. If you'd had another sonographer measure 10 minutes after your first scan, you'd probably get a different measurement yet again.