A week or so before my periods, my body odor changes along with my appetite. Why is that happening?

Lots of hormones. Your period doesn't just involve your uterus. There are many complex hormonal changes happening in your body which affect everything from hair growth to smell to fluid retention to appetite. That is what happens; why it happens is so you can reproduce and continue the species. On a deeper level, if I could answer why, I could probably get a nobel prize!