Hisako, I hope you feel better tomorrow! The writing is good. There are a few small issues, mostly with spelling.

Stomach Flu

I got a stomach flu.
As I was under the weather* and had fever last Sunday, I thought I caught** a cold.
The next day, my temperature was down so I went to work as usual.
However, I had stomach pain while standing and walking so I got a half day off*** to see the doctor.
The doctor told me that I had a stomach flu and he prescribed some medicines****. For the past two days, I have been having trots***** and I didn't have appetite so I just stayed hydrated.
I wanted to attend an English class today but I decided to be absent it would be better to stay home.
Now, I think my stomach is getting better so I will eat something finally and I will go to work tomorrow!

*I recently found the expression 元気のないよう in a book, and I thought of a ダジャレ：「天気のないよう」 Okay, I know, it's not very clever. But 元気 (genki = spirit or health) and　天気 (tenki = weather) sound similar. 

** I know this is a strange expression. We use "catch" (or "caught") to describe getting sick. But honestly, doesn't the illness catch us, and not the other way around? 

*** This is fine, but it would be more natural to say, "so I took a half day off to see the doctor."

**** "Medicine" is used as a plural noun, so you don't need to add "s"

***** TMI!!!******

****** (That's an expression that means それはあまりにも多くの情報ですよ。ಠ‿↼ )

Stomach Flu

I have the stomach flu.
As I was under the wether and had fever last Sunday, I thought I cought a cold.
The next day, my tempteture was down so I went to work as usual.
However, I had stomach pain while standing and walking so I got a half day off to see the doctor.
The doctor told me that I had a stomach flu and he prescribed some medicines. For the past two days, I have been having trots and I didn't have appetite so I just stayed hydrated.
I wanted to attend an English class today but I decided to be absent.
Now, I think my stomach is getting better so I will eat something finally and I will go to work tomorrow!

Stomach Flu

I got a stomach flu.

As I was under the weather and had fever last Sunday, I thought I caught a cold.
The next day, my temperature was down, so I went to work as usual.
However, I had stomach pain while standing and walking, so I got a half day off to see the doctor.
The doctor told me that I had a stomach flu, and he prescribed some medicines. For the past two days, I have been having trots, (<--I don't know what word you're trying to use here.) and I didn't have any appetite, so I just stayed hydrated.
I wanted to attend an English class today, but I decided to be absent.
Now, I think my stomach is getting better, so I will finally eat something finally and I will go to work tomorrow! (<--This sounds better.)

Feel better soon, Hisako! =)

Stomach Flu

 I was under the weather and had a fever last Sunday, so I thought I had a cold.  
The next day, my temperature was down so I went to work as usual, however, I had stomach pains while standing and walking so I took a half day and went to see the doctor.The doctor told me that I had a stomach flu and prescribed some medicines.But, for the past two days, I have been having trots(i don't know what this word is), and I didn't have appetite so I just stayed hydrated.I wanted to attend an English class today but I decided to be absent(or you can say "not to go" instead of " to be absent" that makes you sound like you're not a native which is not what you're going for). Now, I think I'm feeling better so I will finally eat something and go to work tomorrow! 

You did really well in composing this paragraph and although a few things sounded a bit non-native, I didn't want to change the entire paragraph as I'm sure you'll learn how to be more fluid in your writing. I hope this helps :)