So SIL is getting married in November and asked me to be her bridesmaid. So as not to drip feed, SIL doesn't have many friends, none that she would consider close enough to make Bridesmaids so I am her only one. She wanted me to pay for my own dress and accessories that she got to pick, okay fine. My issue came when actually trying the dress on. It was huge on me, literally drowned me and made me look incredibly frumpy so it required altercations. The dress was the only one of its kind (on sale) so we couldn't get it in another size. SIL paid for the general altercations so that it fit me around the bust but still flared out at the bottom and dragged along the floor. I asked SIL if I could pay to get it taken in around my hips so that it fitted better and looked better, she grudgingly agreed. Later that day I get a call from her telling me that me getting the dress altered the way I want is selfish as her wedding has to be a certain style and my dress will then not fit with that style if it's altered and that the day is about herself and her husband not me.
I never implied the day was about me! And now I'm doubting myself because I don't know if I'm being selfish or not. 
Was I? How in the world do I fix this!