Hi there.
I'm afraid I don't have any words of wisdom but we're also experiencing a similar thing. Ours is 18 month old girl and super well socialised-normally greets very politely and more often than not rolls on her back in front of other dogs but recently has become a bit iffy in certain circumstances. Sometimes if another dog goes near her ball when she thinks she's working she'll chase them off with some horribly nasty noises and teeth out (never any contact). Once she did it when she found an animal pelt in a field, and yesterday she had a right go at a dog that tried to take the treat she'd dropped. It's embarrassing and makes you worry. Yesterday was a culmination of events while we were out which I think just put her in a really bad mood. She even snapped at me for telling her off when we got home!!! She is due her season this week and it better happen or I'm really worried.
Thinking about it, I have come up with a couple of things: 1) Obviously the ball/ treat thing is a resource guarding issue so I think perhaps walkies with no ball might be worth a try and no treats with other dogs.
2) This might ring true with you if you have always had a submissive dog- I wonder if mine has just had enough of being roughed up by other dogs. A thousand times she has greeted other dogs with a friendly non confrontational down position and they have proceeded to be really rough with her. It's not uncommon for other dogs to be jumping on her back and body slamming her-there are so many people that don't seem to be able to rein their dogs in when they get too much. She does hold her own ok to a point as she's very fit and muscley but although that weighs her in at 12 kilos she's a really small dog and does get really hammered sometimes. That how our walk started yesterday and I can't help but wonder if maybe now she's a bit older she's just had enough and has decided to take action against anyone who pees her off!?
These cockers are complex beings aren't they? I'm finding it quite hard work sometimes. Like you say, even what should be an enjoyable walk sometimes turns into more of a management exercise!
Hopefully someone will come along and she'd some light for both of us.