Did you take supplemental vitamin D and calcium while on Dilantin?
Is lamotrigine the only seizure medication you take?
Personally regardless of what type of seizure medication I'd take supplemental Calcium and vitamin D.
Tegretol/Carbamazepine, Phenobarbital, Dilantin/Phenytoin and Mysoline/Primidone are all older drugs and the worst offenders for causing bone issues. Many believe the newer drugs as well cause bone problems however the results are controversial. If I were taking newer seizure meds I wouldn’t wait until there was conclusive evidence since the supplements will be beneficial to anyone but especially for women.
Have you had you vitamin D level checked?