Should I feel bad for not going to mine and a friend's birthday trip that my good friend organized?

Okay, one of my best friends planed a trip for me and one of our other friends for our birthday. She was planing for about 5 or so of us to go and rent a hotel for two nights. It was about 55 bucks for each of us to stay in the hotel, so at first I said yes because at the time I had the money. I live with my boyfriend, and we share the mortgage and we share our money. Unfortunatley my boyfriend needed the money I saved up for the trip for a root canal that needed to be done. I called my friend and told her that I could not go and explained why. Then she said that she wanted to take me anyway and not to worry about any costs. As nice as that offer was I told her no because I thought it would be unfair for the others and esp. our other friend whose birthday it was, to pay for everything for me. So I said thank you but no and I told her I would give her the 55 bucks when I can. She got upset. I have said sorry more than once and I dont feel like I need to say it again. Am I in the wrong here or no? I feel like she does not understand the situation I am in....