I understand your concern about drug interaction between both of medications.
And using diphenhydramine together with Skelaxin(metaxalone) may increase side effects such as dizziness,drowsiness, and difficulty concentrating,some impairment in thinking and judgment.
So and currently you are not having associated symptoms then you should watch for such symptoms in next few hours.
It's good to hydrate yourself with plenty of fluids and take some stimulant like coffee,eat frequent small meals.
Also monitor your blood pressure as well.
Avoid driving,watching TV and take proper rest.
You should tell to your family member about your medications as well so that they can lookafter you as well.
If you feel severe dizziness,weakness then consult your doctor or visit ER ASAP for prompt management.
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