You could have a stuck accelerometer. I've purchased 3 drones like this and found an easy fix.
A: Place the drone on it's feet on the ground. Hold the drone steady with your left hand and give it a sharp smack on the ass. You want it to be a sharp smack, but not forceful enough to break anything. You're just trying to unstick that accelerometer, not beat the crap out of it.
or
B: Place the drone laying down on it's back. Hold the drone steady with one hand and give it a sharp smack on it's belly. You don't want the drone to bounce or anything, that's why you're holding it steady with one hand.
Those are two proven methods for unsticking an accelerometer and I've had first hand experience with this one.
If this doesn't work, yes a pixhawk replacement would be next, however don't rush out and buy one just yet. I've a spare lying about I can sell you cheaply. Might as well save yourself some cash rather than pay full price.