Why did my dog suddenly start barking and growling at me?

i've had my dog for a year now and he's always loving and playful but today he was out in the garden and i went to take out the recycling and he was stood on the path with white saliva like foam around his mouth. At which i stop and asked gently what he had. he then jump backward as if he felt threatened and started barking and growling at me he then moved towards me and then did the backing off barking again. his actions really worried me so i went back inside and shut the door on him, he then stood barking at the door so i opened the window to speak to him and he stood wagging his tail at the sound of my voice but was still barking at me and looking at me aggressively after a few minutes he was his normal self again and after 10 i let him back in. he was fine but i felt he was looking at me a bit off. a few hour later i was cooking tea and he started growling at me again i told him firmly to get on his bed and he did but i felt like he was staring at me as if considering weather to really challenge me. its quite a worry cos i have children who he's always been awesome with but why would his behaviour just change for no reason and so suddenly