I currently study BMS (2nd) and also didnt do as well as I thought in my A levels. With friends who do medicine I can guarrantee that medicine is way harder and much more intense.If you like human biology, I would consider continuing, the course is hard like all uni courses but it is very enjoyable, talk to tutors, friends, parents, be honest about how you feel and try and decide if you want a career in biology, if so i would say continue.