Its entirely up to the employer, or the terms of a union contract if you are represented by a union.
Federal labor laws only require 1.5x for hours worked beyond 40 during the established work week. That's it.
There is no legal requirement to pay for holidays, so if a company chooses to offer holiday pay they can pay straight time for the holiday and straight time for the first 40 hours you actually work, which can result in 48 hours of straight time if you work 40 hours during a holiday week.
There is no requirement that you get paid 1.5x for working on a holiday or a day that would normally be a day off. The only reason you get 1.x5 on Monday is because your base schedule is for 40 hours so the Monday shifts put you over 40 and they are legally required to pay 1.5x your regular wage.
There is never a situation where double time is legally required. Some companies choose to pay 2x on holidays, Sundays, or hours beyond a certain point but this is not required and is up to company policy.
In some sense if you work Monday and its a paid holiday you are making either 2x or 2.5x wage because you get your base wage for the paid holiday plus wages for your hours worked (either 1 or 1.5x).
The bottom line is that the company is only legally required to pay 1.5x if you are over 40 hours actually worked, and they are never required to pay 2x, so that's up to them. You'd have to ask your human resources department or read the employee handbook. Your opinion and your GF's opinion about what they "should" pay you is irrelevant.