America: not paid overtime because I worked a paid holiday?

Started a new job and worked last Monday which was a paid holiday then did 40 more hours that pay week working 48.5. My jobs HR says that because I worked that holiday I don't get overtime because the holiday doesn't count towards working 40 hours. So I was paid 8 hours holiday pay, 40 regular pay for that pay week. Ive been working for 17 years and done overtime and worked holidays the same pay week plenty of times and I've never heard of this in my life.