You are probably overworking your eye muscles and are in certain Lind of stress
There is a eye condition called latent squint
which becomes manifest under stress over the body, general debility and eye muscle strain
.this is probably the case with you. Kindly get a complete eye checkup including orthotic checkup and muscle balance check. Improve upon diet, reduce stress and take proper sleep. Eye exercises recommended by eye specialist will take care of the problem.