Here is the US National Library of Medicine page for all Drugs, Herbs and Supplements: https://medlineplus.gov/druginformation.html

The first thing that you must do is to talk to your neurologist/epileptologist about this, OR (if you have a local pharmacist who you trust more than your neurologist), then talk to your pharmacist before your neurologist. They both have access to the same database for medications and ALL of their known side effects, but, since there are more pharmacists than neurologists/epileptologists, then the pharmacist might have more time to get you the answer to your question.

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Here is a link to all of the known side effects of Dilantin (But Drugs.com is public access level, and the pharmacists and doctors have a much more advanced database that they have access to.):
https://www.drugs.com/sfx/dilantin-side-effects...
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Thus, they might be able to tell you specific joints and muscles that the pain has been proven to exist in compared to what is indicated in this public access site.

Or at least which of the links on this public access page of the US National Library of Medicine page of all of the "drugs in the D category (including a pair a for Dilantin), to find if one is the Dilantin that you are on:
https://medlineplus.gov/druginfo/drug_Da.html