Good idea to check with vet to be on the safe side. Like you we have had moments of leash reactivity when Ollie has started growling or snarling at other dogs - this can be to do with them feeling they can't escape the situation so they launch a kind of pre-emptive warning - leave me alone. I think it can make a bit of a difference watching your body language when you walk as well, I feel that when I walk more strongly and confidently and treat Ollie as a dog that has joined me on MY walk rather than me walking to appease his whims that he senses more strongly that I am in control and he doesn't have to be - because I think this can often be a sense when they are walking - 'I'm not sure who's in charge here..wait..maybe it's me...ok, what do I need to do to control any potential threats?' So I walk purposefully with him to heel on a short lead, I don't interrupt my stride for him, or the swing of my arms for him, I rarely look at him - I just do my walk and he happens to be with me and that way he knows I am in control, I will take care of any issues. It also means you can briskly walk past any potential triggers - I often find that it is that little bit longer exposure time that is a trigger to reaction.
I think sometimes changing household dynamics can play a role in their behaviour in the home too..as dogs can gain or lose power according to their relative health, age, closeness to owner etc. - which are all in flux. Of course for many dogs this doesn't matter one hoot - they are not driven to act according to how 'powerful' they are, they don't need to tell other dogs off etc but I think for others it is a more compelling motivator for them.