streaks

I'm fairly new to holdem, but did fairly well playing at low
limit games at the Bellagio and Wynn's. On line, I started
to do well playing 1/2 - 10/20 limit games - but then lost 80%of my gains! I played well for weeks, then had a bad period for weeks! Now I started to do pretty well in NL games
online at 1/2 or 2/4 levels - but again - the last few sessions it seems I'm losing every time. Is this normal? I'm reading books - all Sklansky series, Super System II,
and now Harrington's series (just finishing vol 1). I probably know more than the average online player - but
am puzzled by these streaks.