Does anybody know what's in this girl's head?

She knows that you like her and she doesn't like that idea of it. You seem like the "nice guy". She will use you and get your help as long as YOU allow it and then you'll end up getting back of your head slapped. If I were you, I would keep distance from this girl. 2 notes and 3 text messages will get you being called a stalker. Trust me on this.

Maybe she wasn't mad. Maybe she was just not talking to you because she isn't interested in you and wanted to avoid an awkward situation. It might just be my age group, but I know a lot of girls who simply cut contact with a guy who likes them if the attraction isn't mutual. They feel that would be the easiest way out with the least amount of heartache.
So when you wrote her a note saying you were over it, she felt that a little contact would be okay. Maybe she wasn't ready to slip back into normal conversation with you... but work only conversation was acceptable.

Since she got mad at you for sending her a v-day note, then something important is missing that your not saying, what's important is why exactly was her reasoning for getting mad about the vday note, what was said in it, and why have you two been "off" and "on"?

Aside from that...here are my reasons:

A.) She's dating someone else now
B.) She is confused about her feelings for you
C.) She doesn't want this to go anywhere because you two work together

Those are the ONLY three reasons I can think of as to why she is acting this way. If this is nothing new and she has done this in the past then I would say yes, she prob is stringing you along, but if this is something new then I'm pretty sure its one of the three reasons I gave above. The more info you add the better I can tell you what's going on with her.

Let her go. She is weird. Why would she get mad? I mean if I didn't like someone/had no interest in them and they gave me a V-day card, I would be flattered/honored. It would be sweet to me and if should make your ego go sky high. But getting mad makes me think she is a bitch. She could be suppressing her true feelings by making you think she has no interest in you by getting mad, but who knows that's too deep. I think you deserve better.

I think that you should just leave her alone, obviously she doesn't know what she wants and you don't need to deal with her up and down drama. Just keep a professional relationship with her and nothing more.

maybe she's into women?

She's not interested in you.
You haven't created any attraction and just scared her off with you try hard behaviour and love notes. That stuff doesn't work in real life lol.
Check out link to get an idea of how to create attraction.

Holy smokes, brother, does any sane man ever know what's going on in a woman's head? OK, there is one exception which involves flying buttons, the destruction of your favorite Perma Press shirt, guttural animal sounds, and the sudden attachment of her mouth and tongue to your person like a hungry lamprey. We can demystify that one another time.

You have broken one of the most inviolable edicts of the animal kingdom: Never tap the booty at work! It's akin to cross-species mating, Russian roulette, or voting Republican. All office romances are doomed. And if they end badly, you have no control over the inevitable rumors that follow. Word at the water cooler could be that you're packing more heat than Dr. Manhattan, or that you have a strange wangle on your dangle. The content of your midnight confessions could become the stuff of hilarious office antics. And that's just for starters. Co-workers love to gossip. I can't wait to hear dirt on the misadventures of people I work with.

The rumor mill can damage your rep, ooze into recommendations, kill a promotion, or poison a job reference. You just never know.

OK, so your co-worker attacked you a few times, like a Dane plundering the south coast of ninth century Britain. There's no cause to feel like a ravaged Anglo-Saxon village girl. Don't be so naive. You got reeled in, shared some real satisfaction, and then got tossed back. This is a good thing, Grasshopper. Now go sniffing outside the workplace for a woman who will only damage your heart or your iPod.