I have an iPhone 4S from about 2 years. Till now the App Store never asked me a password different from the one I've set when create the Apple account. Today, for no apparent reason, it asked me a different email while I was trying to update my apps. I don't know this email and I think that I've never typed it, neither set it as my default email. If I go to iTunes it shows clearly that my Apple ID has the correct email, I also tried to verify it and the password is correct. But the App Store keeps asking the password for that different account, and there I can't change it. How do I "say to the App Store" that I want to sign it with a different email?
I recognized that I haven't problems updating other apps, just one: whatsapp. I imported this app from another computer, may this be the problem?