If you want to be less formal than James' answer, this works as well:
I've used my spending money
I've used my pocket money
Merriam-Webster defines "spending money" as:
money for small personal expenses
Note that both phrases can have the connotation that you don't have any money on you, but not that you're broke.
Note that "Pocket money" can have the connotation of being a child's spare cash, that he or she carries around to spend on fun things. "Spending money" can refer to money set aside for trinkets on vacation or the like. Where I live, though, both terms tend to mean "money that isn't tied up in rent or other necessities."
There's also "walking-around money", which is used in the same way, as suggested by Steve Jessop. Dictionary.com defines it as:
money that is carried on the person for routine expenses and minor emergencies; pocket money.
Though keep in mind that there's also the political connotation of
cash sums given by political managers, district leaders, or the like, to grass-roots workers and others for expenses incurred while canvassing for votes or doing other chores before an election.