Is it posible that my baby isn't growing and I'm further along then what a sonogram says?. my periods have never been regular except since may of 09 I've been getting them every month... my first day of my last period was November 5th... in December 09 I missed my period but didn't think nothing of it... I took a test in January and it was positive... a few days ago I had a sonogram done and it said I was 21weeks... but I feel like I'm further along.. I feel preasure like if my baby has dropped... its already difficult to breath or to finish a meal.. I didn't feel like this with my other 2 children till my 7th or 8th month

You could be a bit further along. They will likely do another scan later on and compare. Pregnancies can vary from one to another as well, so it may also be simply that you are feeling things differently... especially with subsequent pregnancies.
Certainly bring your concerns to your doctor and see what they say.