Hakaijuu

Creature! (ハカイジュウ, Hakaijū) is a Japanese manga series written and illustrated by Shingo Honda. Media Do has licensed it for digital release in English.

Plot
You lead quite a nice life... Competing with your friends... Having a childhood-almost-girlfriend, who is coming back to town... All dreams starting to come true... when the ground suddenly shakes and a shelf drops on your head....

Well... You hit your head just now, so it'd be quite... reasonable that everything went nuts and people get eaten before your eyes, wouldn't it?

Official
Amazon link