From my knowledge this is not someone stealing your money ! Please check your final balance on the whole wallet you will probably see it has  correct balance . The other address money sent to is called change address , That how transaction work . Let me try to explain .
When you send money from address (A) to address (B) , another transaction created with the remaining balance on address (A) to hidden address lets call it (C) , The hidden address that got created is also yours because it is created within your own wallet you just can not see it , But when you check the main balance on your wallet you will see the final balance is correct , Some wallet use the same address as the change address some other wallets do not . For example in this case .

you had 200 in your main address (A) you sending 5.5 to another address ( B) , wallet will generate another hidden address and will create a transaction with the remaining balance 194.5 to the newly created address (C) . now you will see your final balance of your whole wallet is correct , But if you check the balance in  address ( A ) alone you will find it is zero . Thats why when people backup there wallet they should backup there whole wallet not just addresses .

Here is an article that explain what I am trying to  say in details . ( https://en.bitcoin.it/wiki/Change )