How to indicate that I've exceeded my planned spending for some time period, but am not completely lacking money

Solution 1:

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.

Solution 2:

I would personally use "I'm over budget for the week" in this case.

Over-budget

(adverb) Beyond the limits of a budget,

(adjective) Costing more than the amount allowed in a budget

(Source: Oxford Dictionaries)

Solution 3:

I'd suggest something like:

I've already blown my budget for this month, and have to cut back.

That works in informal use, but it also can work in a business setting (I've been told by a boss that we need to cut down on photocopying for the rest of the month, because we've "already blown our reprographics budget" for instance).

This seems to communicate all three things that you want: that your planned spending has been exceeded, that you are currently impecunious, and that when the next budget period commences, your resources will be replenished accordingly.

You might also consider "I'm over budget for this month":

Over-budget (adverb) Beyond the limits of a budget, (adjective) Costing more than the amount allowed in a budget — Oxford Dictionaries

Solution 4:

If you're in the UK and you want to be informal, you might say: "I'm a bit skint this week".

adjective
British informal
(Of a person) having little or no money available: