There are some men who chose to pay all the time. It's stupid self-harming because they're taking on the disadvantages of dead sexed roles without having any of the advantages, but there are men who do that. Maybe they're just idiots. Maybe they're following their social conditioning without thinking about it. The reason doesn't matter - such men still exist and they can be upset by a woman insisting on paying her fair share.
Since you chose to establish a precedent by paying for both of you on the first date, she might be thinking that you're one of those men.
If you are, you're a fool who's probably going to be a doormat. Sexed roles, stupid though they are, can make sense if there's some degree of matched sex roles. That doesn't exist any more and good riddance to it. Anyone who willingly accepts the disadvantages of a sexed role when the advantages no longer exist is making themselves a doormat to be walked over.
If you aren't, talk about it with her. Maybe, as I suggested, she has interpreted your initial unwillingness to split the bill as an indication of your position in general. If so, then communication will sort it out. If not and she expects you to pay because you're a man, walk out. A woman who can't handle sexual equality isn't worth speaking to, let alone dating.