I'm on second pregnancy. With both I was sluggishly tired from about 5-8w. I'm 8w now and am finally getting a little pep back in me.
Actually, to be honest, the sluggishness was one of my hints that I was pregnant again! At first I just thought I was feeling "under the weather" so I bought some vitamins... which was a great thing!!!
Then I started having the very light nausea and breast tenderness, that's when I realized I still hadn't gotten my period. When I missed my period I didn't think about it because my cycles have not been regular since I had my first. But the other clues gave away.
The difference is, with my first pregnancy, starting at 6w I couldn't stomach the smell or taste of meats. This time I can, so that wasn't a clue to this pregnancy.