Dog suddenly behaves aggresively/nervously and i don't know what to do, he scares me a little now!?

my male dog is 4yrs old and was fixed 2 years ago. he has always been a really placid and lovely natured dog who has never been aggressive or had a problem socialising with other dogs. we also have a ***** who is fixed who is a little aggresive with other dogs. recently my friend bought her 3month ***** puppy over so i let out only the male dog and ured her he would be fine. she kept her puppy on the lead and when her puppy would not let him sniff her, he went absolutely crazy! barking, snarling, foaming at mouth and looking like he really wanted to hurt it! i have never seen him like this before and it really scared me! since then when i walk him he is really skittish and gets his ears up and is acting weird around any noises, cats, people going past on bikes like he is poising himself incase something is not right? i feel afraid to walk him now, as his behaviour is so strange and i am paranoid he might get upset and i couldn't control him (he is a german shepherd lab x BIG) i am not afraid he would hurt me but now don't want to take him out becuase i feel like he could get set off by anything. he does not appear like he is sick or any other changes in his normal habits. please does anyone have any tips or suggestion on why this might be, and how i can help him to feel more at ease? thank you