Promotion with a Pay Cut

A few months ago I got a promotion at my corporate job. I’ve continued to work diligently at my old responsibilities, as well as taking on several new responsibilities and learning new skills. This Monday I was, without any discussion or notification until my time card was gone, switched from hourly to salary. The salary pay is more than I was making hourly, if I were only working 40 hours per week. But I frequently put in at least 8+ hours of overtime per paycheck, so this switch to salary is actually a $100/paycheck on average pay cut. I spoke to my boss and he said there probably isn’t anything that can be done about it. I wasn’t given an opportunity to negotiate, and it’s unlikely I will be able to cut back my hours to only 40 per week and still complete all my work.

Are my hands just completely tied here? This is the only company I’ve worked for as a “real adult” and I’ve been here for nearly 6 years, so I don’t have much experience as to how these things are supposed to go.