Interesting. It was probably a control issue. Might have been a new DM and didnt want you "floating away," I know I was like that when I first started teaching. Even so, I try to have my students hover as much as possible when they do an exercise so there is more of a real-life scenario. If they are having problems balancing, I will grab a hold of their BCD shoulder to let them know I am right there and subconsciously calms them down.
On a side note, if you were falling forward I'm guessing you had too much weight in your front. Maybe the weight shifted up or you had a lot of weight in your BC? I keep about 10lbs of lead on my tank strap so I avoid that problem. If yours dosent have that feature, you can buy a trim pocket that should do the trick. XS Scuba Trim Weight Tank Pouch WB101 with reviews at scuba.com Keep in mind that if you are diving warm water with not much weight, this will be unreachable should you need to jettison for an emergency ascent.
If all else fails, and though this probably doesnt apply to you anymore, you can always do the Captain Morgan pose! Having one leg back and one knee up is the best way to remove a weight system because it provides a sturdy platform. AND it makes you look cool!
