Assuming no credits were sent to you, you should only have a little over 1600 credits.
You currently only have 644 credits.
So, from this we can assume you purchased 2 songs at 500 credits each.
Could you please inform us of the 2 songs you attempted to purchase?
If you do, we can check to see if you truly are missing them and from there return your credits so you can purchase them again. This is a small glitch that occurs on rare occasions and with the aforementioned information can be corrected.
Comment