Currently 20 weeks pregnant, could I be having twins?

I am 20 weeks pregnant as of yesterday. I have been experiencing everything early. At my 9 week appointment my doctor heard the heartbeat with a doppler, and said there was background noise that could just be my own heartbeat in addition to the baby's. I had an ultrasound at 11 weeks to confirm my due date, I didn't think to ask them to look for a second one. However, I felt the flutters right around 12-13 weeks, and at 16 weeks I could feel kicks. At around 17 weeks I could feel the kicks from the outside, and lately they have been extremely hard! This morning I felt 17 kicks in just under 2 minutes. It's strange, because sometimes they feel like they are from completely different areas, like one down by my bladder and then immediately after one up by my belly button. I realize it could be a punch and then a kick, but there's always a chance they could miss a twin right? About an hour ago I felt strange movement, not like a kick, so I pulled up my shirt and literally watched the baby roll to the left side of my stomach, up by my belly button. It just all seems a bit early to be feeling such extremes, but I guess it could be attributed to me being so skinny prior to pregnancy. I've also already gained 15 pounds, and my stomach is feeling huge lol :) I guess I will find out for sure tomorrow at my 20 week ultrasound but any input before would be nice!