may well just be the fact that you're growing quickly, people differ.
I had twins and was in maternity clothes from around 8 weeks, another person I know was having triplets and didn't get in mat clothes until around 12 weeks....all differs
I wouldn't worry too much though as I have twins and they are a blessing. It's hard work when they are babies but it is actually easier as they get older as they have a constant play mate.
Sam is 1 month old now and I find some things harder with having 1 this time, such as trying to get him to sleep in his own room. Was so much easier when there were 2 trying their new room together!
Sorry,forgot to mention that twins run on my mums side of the family, my nan had a twin brother.
If you ended up with twins because his mum is a twin then it's just pure coincidence. fraternal wins run on the fmale side of the family, hence it's to do with how many eggs you drop, his side of the family can't influence that
Good luck with whatever the outcome