sex then death

i had a dream about a year ago i was 13 or 15 and it was me my sister and a few really hot guys i was going out with one of them we were all swimming around there was a shower in my driveway everyone went inside except me and my boyfriend i was going to take a shower in the shower on the driveway but he followed me in we had sex for the first time (i dont know this guy i have never seen him in my life) later me and his brother were outside together and all of a sudden i felt something wierd like my dog was in trouble so i ran to the road and saw my boyfriend letting my dog out in the highway so i ran and got my dog and pushed him down i let my boyfriends brother hold my dog i banged my boyfriends head against the ground and killed him after that me and his brother hooked up after that i woke up. what does this dream mean?