About Overdrafts and Negative Available balance?

hi i just wanna ask this really important question, so basically my account balance is positive 100 pounds, but my available balance is -90 pounds, i shopped on amazon brought several items which have all been cleared and now im left with 100 quid with two items left to clear, but those two are worth 40 pounds, i checked, and double checked, i bloody have a A Level maths qualification, so how could i possibly have -90 available balance. please need some advice on this strange situation.|||I'm assuming banks in the U.K. work pretty much the same as banks in the U.S. Anytime someone has a positive balance but negative available balance is because there is a hold on a recent deposit. Have you recently deposited into an ATM. If so, there is probably a hold on some of those funds. If not, you should talk to your bank and figure it out.