In Korea, it's already spring but the weather is still cold and rainy at times/ sometimes. 

Today, I fell and hurt my waist (did you mean your hip?) but I'm not feeling much pain.
If I'm sick tomorrow, (I think you're trying to say "If the pain gets worse by tomorrow," since you didn't mention that you're ill) I'll have to go to the hospital.

The accident happened in a matter of seconds because I was already laying down on the floor when my eyes opened. 

I forgot that there will be puddles of water after rainfall so I walked along the alley as I do normally on other days. 

(An alley is a passage way between or behind buildings.)

(An aisle is a passage between rows of seats in a  church/ train/ theatre, or a passage between shelves in a supermarket or other buildings.)

I will be more careful from now on.
My friends, be careful not to get hurt! 

Today, I fell and hurt my waist but...

In Koea Korea, Spring spring has come but the weather is still cold and rainy by time.

Today, I fell and hurt my waist but i'm I'm not feeling a lots lot of pain.
if i will be sick tomorrow If I have pain tomorrow, i I might have to go to the hospital.

To explain, the accident happened in a matter of seconds,
cause because i I was lying down on the floor when I opened my eyes.

(This might sound better: The accident happened so fast, that by the time I opened my eyes, I was already lying down on the floor.)

i was forget I forgot that water stays my sompany on the ground after a rainfall,
so walked up the aisle road (or stairs?) with no differences between the differently than normal days.

I will be careful from now on.
My friends, Be be careful not to get hurt!!