My Poor Back

My back is sore/painful hurt.
It has been started a few months ago.
I found out why it hurts so much and is getting worse. a lot and get harder.
It is because my belly is getting bigger.

I saw someone write about the fact I read that if people gain 1kg in weight, there is a 5kg burden to ones back. my back get burden 5kg. My back has been  got burdened by 50kg for 1 year.
I should lose my some weight.
I start to swimming in the  middle of December.

my poor back

My back is hurt.
It has been started a few months ago.
I found out why it hurts a lot and get harder.

(I found out why it hurts a lot and has been getting worse.)
It is because my belly is getting bigger.

I saw someone write about the fact that if people gain 1kg weight , my back get burden 5kg. My back has got burden 50kg for 1year. (I appologize but I'm unsure what you're trying to say here. "my back get burden 5kg", however, should be written "The burden on my back is 5kg")
I should lose my weight. ("I should lose weight")
I start to swim middle of December. ("I will start swimming in the middle of December" or "I will begin swimming in the middle of December" or "I will start to swim in the middle of December" )

I hope you back feels better soon and that you are able to reach your goal :)