OVER DRAWN ACCOUNT

I have a problem with my account i purchase some staff from shop on 15 dec 2015 and i receive an sms confirming the transaction is successful, and money has been deducted from my account, so on 08 ja 2016 i receive an sms that i have been over drawn. when i went to the bank to check bank statement i see is the same transaction that i received an sms on 15 dec 2015. my problem is why do they send me an confirmation sms and i be overdrawn with something i paid. bad service