hello... i will need to know a few things about you before we draw any conclusion.
1. what is your age?
2. do you have spectacles...if yes what is it's power? do you use your specs regularly?
3.have u had squint(deviation of eye) before or in childhood?
4.do you do a lot of close work lik reading or lot of computer work?
You should get a dilated refraction
done from an eye specialist if you are not using specs.
take rest in between if u do lot of close work.
work or read in proper illumination and avoid reading small print.
your eyes get tired and cant maintain balance so they tend to get crossed.