I don't know what happened

I was spending the weekend at my grandparents house and my uncle that I haven't seen in maybe 7 years (Yea I know what an uncle) anyways I was upstairs on my phone and I heard my grandparents talking to someone so I ran downstairs to go see who it was and I see him his gf and there son who btw I've never met and I got down and as soon as I seen them my heart rate got out of control and I literally felt like I couldn't breath and I just started to break down like I had no control just sobbing and I ran upstairs locked myself in the room and had to try calming myself down it was the scariest thing I've ever experienced