computer not recognized by iTunes, various apps

Lately I've had to sign in to my mac account on several occasions because the app doesn't recognize my computer. Just now, on iTunes I had to 'authorize' my computer as if it were new. This is not a new computer, it isn't even a new OS. I haven't changed anything since the last update several months ago. Oh, and I get emails from Apple stating: "Your Apple ID...was just used to download...from the App Store on a computer or device that had not previously been associated with that Apple ID. You may also be receiving this email if you reset your password since your last purchase." I have not recently reset my password.

Anyone know what is going on? I do remember my password, so I can sign in to my mac account, but it is annoying and I would like it to stop.