After replacing the display assembly, the phone reboots randomly

SO i replaired my friends iphone 6 plus screen twice. The first time, nothing bad happened, ( i used the pro tech tool kit for both). The second time, after moving all components to the new display assembly and putting all of the parts together and rebooting, the phone decides to randomly reboot often and the battery heats up. I'm assuming the battery is shorting out somehow. I don't know if this is just something where i can just replace the battery or if it isn't fixable.