Do I have to pay for my friends birthday dinner?

I went out with an aquaintance for dinner on her birthday and gave her a very nice gift. Another friend met us and said that since she did not bring a gift, her gift would be that her and I would pay for her dinner.
I was a little upset as I wasn't planning on paying for her dinner (we usually go dutch, and I had already bought her something). The other person volunteered me to pay, and I didn't think that was fair.

Am I being a cheapo?