Its tough to guess without more information but.....
I suspect that your app sent the message that you are interpreting as "a push notification pops up saying 'there was an error adding credits to your account'."
Actually, I suspect that it was not a "push notification" but a UIAlert generated by your app after it registered an error in the IAP purchasing process.
I speculate that the error in the purchasing process was that the user had a credit card and Apple need to update its info for that card. Apple asked them to update the information on that cerdit card. When they did that Apple then let them proceed with the IAP. After such an event, Apple is supposed to send two transactions to your app; a 'failed' followed immediately by a 'purchased'. One of two things happened:
1) Apple screwed up - see other postings on this forum over the last few days regarding this problem.
2) Your app screwed up after it got the 'failed' transaction and did not process the 'purchased' transaction that followed immediately.