We got our bulldog from rescue nearly 2 years ago now. They don't have a huge backstory for her except she was mainly kept outside with another bulldog and was pretty neglected. She was pretty docile from the start but we still taught our children how to behave with her so she has never had any reason to get cross at us. In the last 6 months or so she has started snapping at people (never DH or me and never really with the children - that happened once or twice early on so we told them not to sit on 'her' sofa with her and it stopped). At first it was just when people came to our house and approached her and she would put her ears back then air-bite at them. We told people not to approach her (many still did and got told off - by her and us!). Now however she will often go for the ankles of several people who come in the house (DD's friend, gardener etc). It isn't with everyone and I can't tell what upsets her and what doesn't. Sometimes she'll be fine with someone then they'll try and stroke her and she snaps at them. We mentioned it to the vet and he said she didn't seem to be in any pain. She has stopped walking entirely now so is house and garden bound and this happened at the same sort of time that she got aggressive. Her eating is up and down but currently she is eating well (sometimes she won't eat until 4pm but is always begging for human food!).
It's getting annoying really as I can't have friends over in case she scares them and their children! I'm also a little worried she will bite one day - saying that she had a lot of teeth removed at the end of last year due to decay so she can't do much damage!