twin pregnancy- 3 weeks behind; no MS

my lmp is jan 28 supposed to be im 10w1d but baby measures 7weeks. its been already a week since all the symptoms of being pregnant is gone. i never threw up at all but used to have nausea all day excruciating tiresome but its all gone now. i am so worried and dreaded. my OB scheduled me for another U/S on tuesday for their heartbeat as last time there was none for both. im still hopeful and kept myself on positive side. is it normal for me to be 3 weeks behind. I do have PCOS , days of period always changing.