“I Was Rear Ended Today :(”

Ok so the guy banged into me going i would guess 20-30 when i was at a red (I think he was on his phone) so there was a small dent on the trunk but nothing major he gave me a few hundred dollars and we went our separate ways... Now my back is starting to hurt, if it gets worse am I out of luck? I didnt want to go thew insurance cause there was hardly any damage to my car but if my back goes out thats a real problem... Any info??