Aunt Flo flying in? (girls only please)?

I started puberty in 4th grade, now im in 6th. My stomach has been hurting in random spikes of pain and sometimes I feel like I'm actually chubbier than I am (Im actually really skinny, so its a weird sensation lol) I have all the bodily changes the only thing left is my period. sometimes it feels like I have to pee but i dont. i dont really know how to explain it. and since they say alot of friends start around the same time when they're really close, i dunno. Im just suspicious cuz my BEST Friend started a a few a weeks ago and i dont know im not an expert on this stuff lol