I think a lot of it just the shock of looking at the number. 35!!!! I mean you spend you're whole life thinking 35 is OLD and when you hit it well you're suddenly old too. When I turned 30 I spent a few days being reflective about how old I was, had I accomplished enough for being 30, did I make the most out of my 20's, that sort of stuff. I found that it went away pretty quickly when I couldn't avoid my normal routine and it dawned on me that I'm no different at 30 then I was at 29 and that was the end of it. I'm were I'm at in life and that's that.
Have fun on you're birthday and give it a little while to sink in if it hasn't gone away then maybe its time to think about making some changes...but see how you feel the morning after first.
I hope that made even a lick of sense.