Sounds very similar to Honey.
One thing I have found is that being in a position where she had to share treats was fine while the treats were coming, but it created a build up of tension which then showed itself once the treats were away...
With Honey it is a mixture of guarding and fear - it started through guarding, but a couple of bad experiences also led to some fear in specific circumstances... then that kind of mixed into one issue. She's a lot less fearful than she was, but probably more guardy with other dogs. She generally avoids meeting other dogs, but can cope with a quick hello from a sensible dog.
I also find if she has been in a stressful situation, she is much more likely to react to any other dog (no matter how nice they are!) which is worth keeping an eye on.
We've done a lot of careful work on counter-conditioning, but can't use the clicker out and about as it makes her more prone to guarding. She is much better off lead, as she can avoid dogs she doesn't trust...
I would definately get a check at the vets (Honey is much grumpier if she's feeling stiff) and consult a behaviourist. There can be so many subtle signs to read which would give you a much clearer idea of the issue.