iPhone 5 likes to go on Standby and then pop up my lock screen and then come to the turn off slide menu

so my phone has recently had its battery replaced a few weeks ago. well as of this week, my phone goes on standby whenever it feels like it. then likes to come up onto my lock menu and then goes to the turn off slide. i have done the hard restart and turned it off and on. backed up everything and then put everything back on. im running out of options and my plan for a new phone isn't till novemeber