Intrauterine Growth Restriction - ask your doctor to check into that!
Measurements can be off by as much as 2 weeks. But off by 4 weeks? I would continue to seek answers and make sure the placenta is functioning properly to provide your baby with nutrients and oxygen. I don't mean to freak you out because it could be nothing but it could also be serious and you should look into checking out the placental function. I can't remember the term for the condition I'm thinking about. Let me go find it!
Actually it's not possible (AF=Aunt Flow or the more proper term is menstural cycle) to have a period after pregnancy. In preparation for pregnancy, your uterine lining will start to build up. Then ovulation will occur, and if sperm meets the egg and fertilization occurs the egg will start to divide and then finally implant into this lining. If the egg remains unfertilized, it dissolves and your body sheds this lining. Without this, the egg has nothing to implant into and shedding this while pregnant means you miscarry. Just because a female has bleeding does NOT mean it's a menstural cycle. It means she had vaginal bleeding, due to any number of different things. How far along was this ultrasound done?
it is possible to have AF a mth after u may have conceived, so thats what might be the confusion u r going by one date but the docs and going by the ultra sound! if this helps u can go to gettingpregnant. com under ovulation and see the dates matched up!