What Causes Light Periods?

heyy i am only 15 and im a very worried i might be pregnant my breats have being sore n i have not being fellin myself but i got my peorid but they only lasted 2 and a half days on the first day it was a brown colour and i got realy bag cramp that i have never had before and on the second day it was bright red colour and i didnt have cramp n the last day it was red and brown n i had cramp again but not as bad as the first day i have took to pregncy test one a week after sex thn the next 2 weeks after sex but they both came up negitive but i read on the internet you shoud take a test 3 to 4 weeks after but a dnt wont to go to the doctors incase they phone my mum and tell hur why i was their could u please help me so i know to take the risk and got to my doctors or not