Well, I think I know what is happening.
an explanation:
Your new batteries couldnt POSSIBLY become flat within minutes or seconds. What I am thinking is that your battery's arent secured onto the panel which powers the Tamagotchi.
When you open up the back of your Tamagotchi to put the battery in, make sure the battery is tight and secure. To test, simply press the battery down with your finger and wiggle it a bit, if you hear electronic sounds.any sounds at all it means the Battery isnt in tight.
If you cant secure the battery tight enough, undo the second part of the back, (there should be 4 screws) and tape the battery down with masking/sticky tape. Then screw the back on again. (be careful while doing this, dont knock/damage anything)
Note: You may need your Parent/guardian/adult to help you to do this if you dont feel confident doing it yourself.
If the problem persists, you may need a new Tamagotchi (which I highly doubt you will need)
If that works and your Tamagotchi is working fine again, press button [a] or then select 'download' to return to what you last had on your Tamagotchi.
//Haze