After finding out the hard way how important compass calibration is, I was calibrating every time I used my Phantom 3. Yet last week, I calibrated the compass, set it back on the ground, took off and hovered at about 4 feet up. Everything was perfect. Climbed to about 25 feet and it went crazy, like the compass wasn't calibrated. Drifter over a busy intersection so I couldn't land... eventually crashed and I'm out $$$
Here's my theory. I got out of my car and was setting up next to a nice park with a big stone wall around it. My calibration was done holding the UAS about a foot over that wall. Chances are pretty good that the wall could've had iron in it, and as such, affected my calibration. Same with my car being parked very close.
So when I took off and hovered 4 feet up, I was still close to the alleged magnetic field interference. When I climbed up another 20, I was not. And now my compass was out of wack.
You'd have to look back at the details of your last flight to see if this could be your problem or not. Good luck, I wish you well.