iPhone 5s has beefing shutting down (blacking out) randomly no matter what battery percentage its on

ive had this iPhone 5s for about an year. It never gave me any shut off problems But for the past 2-3 months my phone just shuts off randomly. I would just be using my phone then it's blacks out in the middle of whatever I was doing. I would hear a little sound the second it goes out. I would then just turn it back on no problem. It would usually happen when I'm using the app "snapchat". But when I deleted the app it did the same thing. It once happen while I was on FaceTime and then it wouldn't shut back on. I have no clue what's going on. I went to my carrier (sprint) and I was told to go to an apple store. and just last night I charged my phone to 100 and I unplugged my charger and it shut completly off and would not turn on. I reset it and all. I had to unscrew the phone myself take the battery part that plugs in and out it back and it worked again. But I'm still getting these random shut downs. I would go to apple but I have no money to for to repair it or get a new one. What is going on?