About twins.

So my midwife has only done the Doppler at my appointments. Once she finds the heart beat we listen a bit an then that's it. She doesn't check for a possible 2nd heart beat. Idk why I feel like I may be having twins. I feel too big for 12weeks. Is it possible to not even know that your having twins until you finally get an ultra sound with for me is at 20weekd.