Well, according to the breakfast crew on Virgin Radio.... (not that I'm holding them up to be paragons of the English Language, as that's the last term I would apply to them!), a hottie is:

a hot flush/episode brought on by the placing of one's foot in one's mouth (bad euphemism in the UK at the moment, but you know what I mean...)

This then leads to the phrases 'Are you having a hottie?' or 'Looks like you're having a bit of a hottie there'.

As I say, this is not my personal definition, just hearsay - I'd have gone with the hot water bottle option!