Why does my iPhone 4s keep turning off while I'm on it at random times?

[Alexandra]

Like today I was checking my facebook and I glanced at my battery it was around 50% then I checked my Instagram and all of a sudden it turned off and when I tried to turn it back on it showed the dead battery and I had to plug my iphone in a charger but I didn't have a charger on me so I waited a minute or two and tried turning it back on. It turned back on and showed the battery at 30% so I left my phone till I could get to a charger. So my mum was running a bit late picking me up so I called her and my phone turned off and this time it didn't turn back on and my phone wasn't even less than 20%. What is wrong with my phone this has been happening heaps lately nearly everyday!