I Dont Agree With The First Comment , I Had The Same Thing With My Daughter, She Was 9lbs So Yes If The Baby Is Big And The Baby Stretches Its Possible To Feel Movement Like That.
AND My Mother In law Was Pregnant 6yrs Ago , They Told Her She Had One Boy..... At 35 Weeks When She Started Labor They Noticed Baby B ANOTHER Boy Hiding Behind Baby A .. They Never Picked It Up. And She Wasn't Very Big At All! Baby A Delivered Naturally And Was 7lbs , Baby B Was CSection And 5lbs. Explains Why She Went Into Labor Early As Well , So Yes Its Possible For Twins To Be Overlooked... Rare But Possible If Baby B Is Hiding.
Yes you're over thinking it. They would've likely noticed by now if it was twins. Plus it's not uncommon to feel movements in different spots depending on how the baby is positioned.