Did I have my period?

I am almost 13 and i wear a 34a...almost a b. And about 2 months ago i got this big brown spot in my underwear, but it only happened that one day. I did this thing where i hang a pencil over my wrist and it goes in a straight line if i will have a boy and a circle if i will have a girl...and it worked...but then two of my friends who has never had anything like i did and they tried the pencil trick and it didn't do anything. i am not sure if i had it or not!!!!! please help me i am afraid it will happen ago when i am swimming.