Help!

Im 22 and this is my second pregnancy. My husband and I just found out we are having twins! We have no idea how this happen. Neither him nor me are twins, nor our parents or grandparents. How did we get two babies! I'm excited scared and nervous all the same time. If some one could shed some light on what might have happen? And what should I expect! I'm 7 weeks and 3 days :) thank you