Is it my fault if a biker rear ended me?

while heading for an intersection, i thought no cars were gonna cross so i didnt slow down but all of a sudden a car crossed in front of me so i suddenly braked to prevent a collision. It was okay i wasnt that close to that car but out of nowhere instantly i hear a thud and it turns out a bike hit me and the guy fell. he was okay but it wasnt a good day for him. i felt bad, it was night I didnt know he was gonna be behind me.