Brief Answer:
multiple causes need assessment

Detailed Answer:
Thank you for asking

I read your question and I understand your concern. unilatetal dilation of pupil is usually benign and is called anisocoria. It is a common condition, defined by a difference of 0.4 mm or more between the pupils of the eyes.It has various causes

1) Physiological anisocoria: About 20% of normal people have a slight difference in pupil size which is known as physiological anisocoria. In this condition, the difference between pupils is usually less than 1 mm.
2) Horner's syndrome

3) Mechanical anisocoria: Occasionally previous trauma, eye surgery, or inflammation (uveitis, angle closure glaucoma) can lead to adhesions between the iris and the lens.

4) Adie tonic pupil: Tonic pupil is usually an isolated benign entity, presenting in young women. It may be associated with loss of deep tendon reflex (Adie's syndrome). Tonic pupil is characterized by delayed dilation of iris especially after near stimulus, segmental iris constriction, and sensitivity of pupil to a weak solution of pilocarpine.

5) Oculomotor nerve palsy: Ischemia, intracranial aneurysm, head trauma, and brain tumors are the most common causes of oculomotor nerve palsy in adults. In ischemic lesions of the oculomotor nerve, pupillary function is usually spared whereas in compressive lesions the pupil is involved.

6) Pharmacological agents will cause anisocoria if instilled in one eye. Some examples of pharmacological agents which may affect the pupils include pilocarpine, cocaine, tropicamide, MDMA, Dextromethorphan, and ergolines.Alkaloids present in plants of the genera Brugmansia and Datura, such as scopolamine, may also induce anisocoria.

I explained six major domains of anisocoria there are plenty beyond the scope of this discussion. You should consult an opthalmologist and let them help you out here.

I hope it helps. Take good care of yourself and don't forget to close the discussion please.

Regards XXXXXXX

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