Why would a shower with new pipes put in two days ago with no leakage start to leak tonight? It is by the handles to turn on and off the water behind the wall.

Is the leak on the outside of the wall or inside..?

Why would a shower with new pipes put in two days ago with no leakage start to leak tonight? It is by the handles to turn on and off the water behind the wall.

From your post I take it that you've installed a rwo handled shower valve,(see image) that's leaking when it's turned on. If the leak's coming from the stems the all you have to do is remove the handles and the chrome cover plates. There will be a brass nut on the stem. This will be the bonnet packing nut. Turn the shower valve on and snug the nut down until the leak stops. Do not overtighten. Stop when the leak stops. Let me know how you make out. Tom