No, don't call 911 right away, grab your Glock first. In your situation you are better off calling your grandfather anyway who can be over there immediately. You are also better off grabbing your gun than you are on waiting for an officer to show up because that could take way longer than it will take someone to come in and hurt you.
I don't think you made a mistake in going back downstairs to get your gun. However, you should probably have stayed downstairs until your grandfather came over and then y'all could check the house together. I probably would have run to grab the gun the minute I saw the guy instead of asking him what he needed. In fact, no reason to even ask the guy what he wants. Just wait til he goes away and have your gun ready in case he comes in. Yes, call grandpa while you wait.
You can call 911, and maybe you should if the guy looks like he's up to no good, but get your gun first!