In the creation kit I was making some simple modification to player stats. I took a break and saved and exited creation kit, when I went back to continue modding and I tried to save it to the same esp it wouldn't let me. I keep getting a message saying File:"Whatever.esp" is a master file or in use please select another file to save to..

When you restart the CK and select your mod to edit it, you need to set it as the "active file" or else the CK will think you're making a new .esp.