Defective battery or nothing to worry about?

They swapped my iPhone 4 for another one last Friday at the Apple Store. I had no problems restoring from the backup I had on iTunes. Like I usually do, I let the battery drain, and this morning I fully charged it. I have been using during the day the phone without any problems. This afternoon I when to take a nap and after maybe an hour I took my phone to check my email and when I pressed the home button, it did not wake up. I pressed the power/sleep botton with no luck. I pressed both the sleep and Home buttons and nothing happened. I plugged in to the wall and did not started charging neither made that chirping sound. When plugged to the USB to the computer iTunes did not recognized neither the"chirping sound". I call Apple and they toll me I should make an appointment with a Genius at an Apple store which I did. However, after trying several more times, the hard restart (Sleep and home button) the iPhone started and it has been working fine. This had never happened to me and I don't feel comfortable because it may happen again. It may be a defective battery. Had this happened to somebody and there was no problems later on with the device? What would be your advice. Thank you.