How far along in my pregnancy am i?

i'am so confused by this. ok so i went to my wic appointment and was told that i got pregnant december 10th. i'am 16 weeks, and im due on the 4th of sep. she said that i turned 3 months pregnant on the 10th of march. well if you count 40 weeks from the 10th of december, that puts my due date at sep 16th. but my due date hasnt changed. i have no idea how far along iam and it is really bugging me. i thought that i was 14 weeks right now but she bumped up my weeks by 2 weeks and bumped down my month by 2 weeks, how in the world?