I have back pain again.

(good)

(While working, I picked up a load of stuff from the floor and my back pain returned)

I didn't feel any pain recently,I was careless at that time.

( although I don't feel any pain now, I was careless to hurt it again at that time)

Since I didn't want it to be worse, I had half day of in the afternoon.

( since I didn't want it to get any worse, I took a half day off from work that afternoon)

I hope it get better soon.

(I hope it gets better soon)

I have back pain again.

そうだったんですね。(You taught me that ;) But I feel rude that I'm not telling you "I'm sorry about your back")

I have noticed Japanese speakers often forget to but a space after the comma. It's probably because in Japanese writing, spaces aren't used. but it's important to include a space after any punctuation mark.

"This time" or "that time" - Because you are talking about a specific time, and it was very recent, I would use "this time." But if I had hurt myself two weeks ago, I would say "that time." 

I hope you feel better.

I have back pain again.

I have back pain again.