With first ones anything is possible! lol
As soon as I found out I was pg with #1 I seemed to eat more. I guess I thought I needed to eat for 2. Are you eating more then normal? Can it be weight gain from that? If you are really tiny, you said a size 0, it is a big possibility that you are already showing. I would not rush into maternity clothes right away. You are still pretty small from what I understand with the size 0's just being snug. You could do the rubber band trick with you jeans. Just thread a rubber band through the button hole and around the button. I did this with all mine. I did that and wore bigger shirts. With the first 2 I didn't even own maternity clothes. Just bigger normal clothes. Good luck with your pregnancy. Hope all goes well.