twins?!

Tabitha
I'm beginning to wonder if I'm either further along than we think (dr apt 2-7 for confirmation) they say currently a little over six weeks or if I'm having twins. What's the chances of twins they run in both my family and my husbands family? What are signs and symptoms of twins? I should also say I'm still under weight wise of pre-pregnancy but my belly is getting big quickly.
Sorry for so many questions first time pregnancy and we've told no one so I only have you guys to turn too for questions