ALL your income is taxable by your home state of MD. Submitting Form REV-419 to your PA employer as described by @Critter should cause them to cease withholding PA taxes (and hopefully to start withholding MD taxes).
If no MD taxes have been withheld for you this year, and/or if your employer cannot or will not withhold MD taxes, you should consider making quarterly estimated tax payments to MD. Here's a link to MD's page on estimated taxes: https://taxes.marylandtaxes.gov/Individual_Taxes/Individual_Tax_Types/Income_Tax/Payment_Information...
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
