Do I have to pay for lunch if the other person offered the first time?

Me and a friend have gone out for lunch a few months ago. My initial assumption was that we would either split the bill or each pay for our own part, since that is the standard in my country (the Netherlands), but at the end she asked if she could pay for me. I asked if she was sure and say that she didn’t have to, to which she answered that she was sure. Thus I gratefully accepted.

This week, we will go out for lunch a second time and I’m wondering, is it socially necessary or polite that I pay this time?

I’m not really looking forward to paying for the whole thing, but I don’t want to be rude.

edit: I did pay for a snack when we went to an amusement park with her, but the price of that was significantly lower than the lunch (we go to the same university and the tickets were payed for by the university).