Sorry mate to hear this, sounds like a tricky situation. For now I would wait for the guy who gave you permission to sort this out for you, it's in no way your fault as others have said, maybe go round to landowner and properly introduce yourself and be polite stating you didn't realise. Have a sit down, the landowner may be more approachable instead of standing in the field with a gun.
I don't envy your situation so the best of luck mate.