Is it normal for the hurt from a past relationship to linger even when you've found someone new?

A few months ago, I was hurt the worst I've ever been by a guy, and the bad ending took its toll on me. I got to the point where I didn't trust anyone and couldn't get the situation out of my head. Much to my surprise, I eventually learned to trust someone again, my current boyfriend. He was the first one to change my negative feelings, and I felt that I had completely moved on to him. We've been together for a month or two, and things have been great so far. but recently I've had a fresh wave of hurt from my past come up. I don't know how or why, but I've been feeling listless and vulnerable like I did during the recovery process after my last relationship. I flinch when people mention him, I get shaky when he's in the same room as me, I randomly reminisce (which never ends well). I haven't talked to anyone about this because I find it all silly and unnecessary, I have a great guy now who is nothing like the last one. So why is this happening? Is it normal considering the past is still relatively fresh?