when you captured 8...cxd4 then he could have also hitted it with his c pawn rather than the knight, making his center strong.
the next move proved this point as his e pawn was isolated and was a weakness
he also opened up one of the center files for the rook..
here is a game i played on satuarday the time controls 36 move in 90mins and 30 mins finish i used 116 he used 15