Doggy drama at the cafe!

[Phteven]

I don't know what to think. My dog is pretty feral about other dogs but she is usually very tolerant of people. My boys are rough with her as children often are and she is always gentle with them although she will give a warning growl to let them know if they have pushed her too far. When visitors come she barks at them but generally after that she is fine. She is not out-going. She is a one person dog and I am her person but she is always tolerant of pats and once she gets to know you she is a lot more friendly. She always has a wag for my siblings and their partners.
Most mornings we go to a cafe for a coffee. They know her there and she always sleeps under my chair and ignores anything other than other dogs.
Today I was having my coffee and she was under my chair as usual when suddenly all hell breaks out - she was snarling and snapping at an older man who was swearing and snarling back. Then his wife came over and he said watch for the dog and she asked was he bitten. He said Nearly.
Then he was complaining to the owner about her. The owner knows her and she knows that she is usually very placid but since the whole conversation was in German I don't know if she pointed that out.

She's never done anything like that and now I'm kind of worried that she will do it again - she is a kiddy magnet. They come from miles to pat her because she is small and cute and now I suddenly feel worried that she might hurt one. Most days our walk is interrupted at least once by a small person who wants a pat.

Part of me thinks that the problem is the old man - the way that he was snarling at Bella almost simultaneously when she started going off and the way his wife asked if he had been bitten in such an everyday tone of voice makes me wonder if he has issews with small fluffy dogs and this kind of thing happens often.
Another part of me thinks that maybe the problem is that Bella is more psycho than I ever thought - I don't want to be one of those owners who sticks their head in the sand and pretends that their feral horrible dog is normal but like I said this has never, ever happened before.

What can I do? She is due for her vaccinations and checkup in a month so I thought I could go tomorrow and ask the vet but in the meantime..... has anyone else's dog ever freaked out at someone for no reason? Did they do it again? My Dad's dog went for someone once much to our embarrassment and later it turned out that he had given us a bouncing check but I don't want to take it from that that all dogs who suddenly act snappy are loyal helpers saving the world from fraudsters and mean people.

I love my dog but I'm not prepared to keep a dangerous dog or ask anyone else to adopt her