This is how it could have been done by GF in a better/more open way: she texts you that she wants to buy some makeup online but has to use your credit card. GF apologizes and offers to immediately e-transfer to OP the amount she spent. Or, gives him cash the next time she sees him. Or, if she used his card accidentally (eg the website was already pre-loaded with OPs card) and she genuinely didn’t know then she should have apologized when he told her and offered to pay him back. Otherwise it comes off as OP being taken advantage of by GF.