I have a Pregnancy Question?

Ok, so ive never gotten my period, i dont think, and 10 minutes ago i found a pair of shorts in my hamper. I thought my mom bought them for me so i tried them on. I tried them on with my underwear and jean shorts on, and i tried them on backwards, so the part where the should be was facing forwards. After I took them off, I realized they were my brother's boxers!!! (He just turned 9.) I am 12. Is htere any chance I could have gotten pregnant? Could his sperm (If he has any?) Creeped past???? Please help me I am super-scared!