I think my girlfriend's stealing from me. What should I do?

Secure your debit card(s). Change your pin. Notify your bank. They can help you get to the bottom of things. Your statement should tell you when and where the withdrawals occurred. Don't confront her unless you're sure. You've only been together 4 months. There is probably no "understanding" between you about shared monies. If she's stealing then you should break up right away. Decide if the amount is worth pursuing legal action. The "love" isn't worth the aggravation of being with a thief.