This is happening to my best friend right now. Its her 3rd kid and the doctor said that it was normal (I don't like her doctor but that could because my doctor is very very hands on) I can see with the fact its her 3rd kid and she normally has bigger kids to not worry! I do know though she is check each week to make sure the baby is OK! Sometimes the baby's that messuare bigger develop faster! My niece is an example born 45 days early because she was already fully developed but caused problems for her twin sister who wasn't!
those measurements areoften wrong.I was measuring 4 Weeks more but baby is the perfect size.I wouldn't worry too much.
If it was an external measure if the uterus then it can easily be wrong. Depending on the position of the baby you can measure higher or lower than what you're supposed to.
Those measurements are not always right. If they measured the outside of your belly you could have more water then you did before. When I was pregnant I would measure smaller for some reason my body carried less water for the baby. I know they would always say I was behind what I really was because of this until the ultrasound. You may just have more water .. or the baby is a little bigger! =) I would not worry .. good luck!