Yeah, it's probably the monitor. But you need to make sure before replacing it. You just need to connect your PC to a TV(since you don't have another monitor) to see if it still gets artifacts. If it doesn't, it's the monitor. If it does, it's the GPU.
By the way, you said this happened when you tilted it down. Does it continue if you tilt it back up? And does it happen constantly, like when you're only on the desktop?