I was flying my Phantom 3 Standard, and when I connected it to the remote controller, it said on the DJI GO app that it was going further and further away even though it wasn't even moving. I have never had any crashes ever, yet it appears the GPS is messed up.

The GPS unit is usually extremely reliable.
Can you provide amy more information?
Did this happen just for a couple of seconds, is it still happening etc?

Hey there, does the issue appears recently? Could the drone be controlled by the RC? Please kindly offer a short video to illustrate the issue clearly, we'd like to check the exact status.

Yes, it could be controlled fine. It's just that it must have had some calibrating issues or something because it works in other locations, just not this particular one. I've calibrated it several times with repeated success. I

"I was flying my Phantom 3 Standard, and when I connected it to the remote controller...."
Are you starting app while on the ground and waiting for home point to be established? Your first post wasn't clear about that.

You can expect up to 30 feet separation between the displayed aircraft location and it's actual position. That's the nature of a GPS system that's marginally better than the one china puts in your cell phone. With the drone parked on the ground, it's common to see it's location move around on the map to a certain extent. I See this particularly when my drone is parked next to a structure like my home..... in fact, the GPS signal can be so weak that merely standing over the drone will make it toggle the ATT'i mode.  

The GPS signal is extremely weak at ground level. Many times I have watched the parked aircraft's position on the app's map wander slowly but definately around to a certain extent.
An RTH to the recorded Home point could land the drone 20 to 30 feet away from the actual take off location. So weak is the GPS signal that merely standing over the drone can make it switch into ATTI mode.

On the good side, the drone's GPS accuracy is amazingly good once the drone is up in the air and no close structures, trees etc are blocking the drone's view of the real horizon.

You did not mention how many satellites you were showing on the display.  You may be in an area that does not have a clear view of the sky.

Sorry for the long reply, I have full satellites at all times. It's hovering right in front of me, not moving, so I know the GPS stabilizes it. It just starts saying it's going further and further away even when it's not moving.  I'm only worried because if I need to RTH, it might think home is 20 feet away and land in a tree.

Go out to an open area and take it out further and see if this happens.  Test RTH too.

Ok. Thanks

Wait with the take off until the app tells you >home point has been updated,please check it on the map< then you can take off. The real landing place after a RTH should be inbound 10 -15 feet radius around the recorded home point,not more. The P3S uses GPS only. The other DJI copters uses GPS and Glonas,have more locked satellites. Because of that the P3S is not so accurate as a P3A/P P4 or Mavic.