Expression "it's on me"
This expression usually means that you will pay for something, e.g., after lunch with your friend you can say when you go to pay "it's on me" which means I am going to pay for myself and the friend.

Recently, I have heard the expression being used in another way and I am wondering if there is now another meaning to it?
The new way is that it means "it's my fault".

Question

Expression "it's on me"
This expression usually means that you will pay for something, e.g., after lunch with your friend you can say when you go to pay "it's on me" which means I am going to pay for myself and the friend.

Recently, I have heard the expression being used in another way and I am wondering if there is now another meaning to it?
The new way is that it means "it's my fault".

Answer

It's not a new usage in AmE. It's been around for years.

Answer

I didn't realise that this was a new way to use it?

Also, I occasionally hear it used by someone taking responsibility for a task, e.g. they're allocating it to themselves.

Answer

I agree that it has been used in the US for a long time. :)
  However, I have heard these phrases more in conversation:  "That was on me" or "That one was on me".  I rarely hear "it", and usually hear "that".

Answer

Thanks for the comments, so to sum up, the meaning of "it's my fault is an American English term, , but not used particularly in Europe.
I'm Irish and I certainly have never heard it being used in that sense there.

Answer

Hi, folks in the United States use it for that purpose, e.g Akon: "Blame It On Me" and colloquially.