I have the Blockchain app on my iPhone which I've been using regularly for years but which in fact I haven't used for the last 4 months. It currently has around $2000 worth of BTC in my private wallet. I have just gone to make a purchase for goods of around $50. I click on Send, select Private Key Wallet, enter the recipient's address but as I type $50 it tells me in red that I am trying to send an amount over my limit. Can anyone tell me what's happening with this?