<human>: Can you condense the text into a few key points?

Both in our mid-20s.Have been dating my GF for 3.3 years. We were talking about me claiming  her on my taxes, since one of our friends does it and I looked into it, and we meet all of the qualifications. Plus it was my GF's idea. 

Also, I pay 100% of rent, and utilities. My GF pays for some things here and there. And her mother lives across the entire country and she hasn't lived there in a long time.

1 day before submitting my taxes I hear that her mother claimed her/my GF, and was sending down a couple hundred dollars $150 and that she got their taxes back. (She received WAY more than just a few hundred...)

This seems so stupid, but at the same time, I don't make that much money, my GF is still in school for another year. My GF hasn't lived with her parents in the longest time, and clearly aren't paying our bills. 

Why does this annoy me so much? Her mother, sending down a couple hundred, thinks she's being "sweet" when in reality it's not 100% of the amount. I find it slightly tacky. But then again, I feel shitty just posting this, but feel like the mother didn't have much of a right. My GF and I really watch our pennies, and it really could've helped.
<bot>: I pay for most of my GF's life but mother claimed her on their taxes. We've been living together for almost 2 years.