No you’re probably not the only one who has felt something seeing their ex. Some would even say that it’s normal especially if what you guys had was something special and extraordinary.
For me I believe that we can never forget about people from the past. Even if we say we’ve moved on, there will always be a part of us that came from that other person and they will always be with us. That is why I think it’s normal to feel something when you see people from the past. Be it regret, love or sadness you just have to accept it and move forward with whatever those feelings are. What you are feeling is valid but it does not mean you should act on it.