Bipolar or completely normal?

My son is 15 months and I swear, he can switch emotions so quickly. Like he will be laughing and smiling one minute, and the next he is having an absolute meltdown, screaming and kicking and throwing himself on the floor. If I tell him no even once, he bursts into tears and hits me and then jumps off of my lap and throws a giant fit. I know toddlers throw tantrums and all that, but it's insane to me how fast he switches. I try and talk to him and tell him that it's not okay to do that. I try and make him feel better. I feel like I've tried everything and he is out of control.