Tommy98 Please check here - heteroforie.webnode.cz
If the optometrist did not find anything, then that unfortunately doesnt mean its still not heterophoria. You can try the home test and maybe find more specialists to try.
It may be stress, but in a different way than you think now - long time under pressure or sudden stress can make heterophoria manifest, as the strain is already too much and under bigger load your body may just give up and let the eyes "slide".
Also a lot of people here are looking for neurological solution - heterophoria IS a neurological issue as well. Its a combination of badly set eye muscles combined with how the visual center in the brain processes stimuli.