Sounds to me that she has learned to equate buy stuff with feeling good (aka shopping for a good feeling). She may be buying things for you as a way of expressing her "love" for you.

i agree with ~Cristina on this. You need to keep your finances separate. Her reckless spending may ruin your credit rating.

You also need to start refusing the gifts she is buying you. Accepting gifts that you know she can not afford simply reinforces her behaviour. She needs to know that you love her for who she is, not for what she can buy you.