With the subsequent pregnancies you will show much earlier generally. Happens to me too. Usually twins will be detected in ultrasound pretty easily in early pregnancy. You will see either 2 sacs or 1 sac with 2 fetal poles in there. Unlikely nowadays doc will miss it.
I would wonder the same , I'm 20 weeks now and have soooo much movement ! Can feel moving and kicks way down the left side of my tummy and up the right side , my 13 weeks scan only showed one baby but my first scan showed a cyst aswel, I'm sure there is two! But last scan at 13 wk was very clear