It seems the extra money I thought I had in there on Monday wasn't a mis-calculation on my part, just a late transaction going through.
I paid the next months storage on the storage unit on Saturday, but for some reason it didn't go through to my bank account until yesterday. As a result my calculations were out, and when I took money out of the bank on Tuesday it ended up going overdrawn yesterday when the transaction actually went through.
I get paid tomorrow night so it's only temporary, but it's going to end up costing me an extra $60 ($30 overdrawn fee yesterday and today). Oh well. It's my own fault for not keeping track of what I was doing with my money.