from what i have read, i dont believe you could be pregnant, but. there is a great book i read about that certain topic in it, i think its called girl stuff, going by your age, 20.. you have had your periodn for a few years?
becuse for the first two years or so, that seems to happen a lot. did to me.
sorry if i am of no help, but, i am just saying what i know, with the hope of helping in some way :p