What do I do about my potential stalker?

I'm a senior in high school. The last few weeks I've been recieving gifts of skittles in my locker. Then my favorite energy drink. It's obvious this person knows what I like. Then I found a $10 bill with a note saying "to help with gas money". How did they know I was running out of money?! This is getting outta hand. And worst of all: It's making my girlfriend EXTREMELY angry. She swears it's not her. and I kno that is true. Because she also has very little money. Whever it is, they kno a lot about me. What do I do to find out who it is? And how do I calm down my girlfriend? haha