Am I being used?

I moved in with my girlfriend in an apartment down from her mum which her mum owns. I pay for everything for my girlfriend even though it’s not my responsibility as she is only 19 and still studying. This month we went abroad and I ran out of money and her mum asked for the rent. I pay the bills also groceries. I said I have some money to give but not all of it as I had some expenses and she said let everyone manage themselves and what can I say. I feel like I have been used just to take care of her daughter and my money, I am kind of lost I am not sure what to do
Am I being used?
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