Why would i have double vision?

Many reasons: Double vision is due to lack of coherence of eye muscles, and can be due to problems with nerve locally, eye muscle weakness or fatigue (myasthenia gravis), problems at base of brain (multiple sclerosis), some medications (usually rare), infections (nasopharyngeal), problems in cavernous sinus, orbital areas, retrobulbar sites. See an ophthalmologist and get an answer.

Mono vs binocular: One way to test yourself: cover either eye with your hand. If the double vision is still present with one eye occluded, then this is "monocular diplopia" and rarely neurologically threatening. Most causes of monocular diplopia are optical: astigmatism, cataracts, macular pucker etc. However, if the double vision disappears with either eye covered, this can be more serious & need to see eyemd.