iPhone 4s lines in display

I just got an iPhone 4s today which I purchased from a store. The device has been used for more than an year so there is no warranty. I didn't notice anything suspecious while checking the phone before making purchase. I was using iPhone 4G before so everything looked similar other than some new features like Siri, Springboard etc.

After getting home, I setup my new iPhone. I downloaded and installed Facebook app. I noticed something different while opening pictures on Facebook. There was a band/thick line with every text or facebook button outside the image. I restarted my iPhone and I noticed that same band/thick line appears with the Apple logo while start up.

Simple question:
- Is this something normal? Did I make some mistake while doing settings? Is this some hardware problem?

I tried to take a screenshot of the image. Those bands/lines appeared in the images in my phone but when I downloaded them in my laptop, there was no such thing. The images were just normal. I have taken those images from my iPhone 4 to show your guys.

Please have a look and advice.

N.B. I am using iOs 6.1.2 (10B146). Rest of images/videos/displays are normal, just like iPhone 4G.

Thanks in