I don't use blockchain.info so I can't speak from experience, but it's normal for online wallets to assign multiple addresses to one user. The old address you're talking about is probably still associated with your account. For additional reference, this is from the blockchain.info FAQ:
What is an address? ×
Addresses are strings of letters and numbers, that are used to send you bitcoin or ether. For increased privacy, a new bitcoin address is generated for every transaction. Find the address for your next transaction by clicking receive in your wallet.
If you've used the old one, you were likely assigned a new one.