We rescued our dog from the council strays about 2 months ago. She's about 15 months and we think she's a collie/staffy cross. She's been fine most of the time, just the odd bit of bad behaviour - barking at people/dogs/cats/cars on walks, chewing bits of the wall etc when we're out - but nothing we didn't really expect from a rescue dog.
But this evening, out of nowhere, she bit my wife's hand - not quite hard enough to break the skin but enough to leave bruises, and it's still really sore.
My wife was only cleaning 'eye goo' off the dog's face the same as she's done loads of times before, and the dog's never minded her face being touched in the past, infact I was stroking her face only 10 minutes before she bit!
We really dont know what to do now...
We dont feel like we can trust her anymore, especially if she can be as unpredictable as that without being provoked.
Is there anything we can do?
We'd feel so guilty giving her away again as it's already happened to her once - and I'm sure nowhere would even take her knowing that she'd bitten someone anyway!

If anyone has an advice for us we'd appreciate it...

Thanks