Rear driver side tire suddenly has negative camber

I was driving one day and a person honked behind me and pulled into the gas station and pulled in and told me that my back driver side tire was leaning out words like it has negative camber, but it's only the driver side left tire. But in within a week it progressively got worse and lot more noticeable. What could be causing this? I'm on a budget and I cannot afford all the parts that I've researched that may be causing this. I need help diagnosing.