So I'm not sure if you'd call this a bug or what, but I was in the den at Goodneighbour, and finished the dream sequence for Kellogg's memories.

I went upstairs, to see what there was too see. I went to Irma's computer and although it was red, no one was around so I figured it safe to log in... man was I wrong, everyone was hostile afterwards.

So I go to reload the nearest save point, I believe it was a save right after getting out of the memory chair doo-dad. Everyone is still hostile though....

I had to reload further down the line, but is that something that happens regularly, or is it supposed to at all?

Should have tried restarting the game. Reloading a save doesn't always reset everything if it's in the same cell that you were in before you reloaded.