Yes, It happens
Cass,
Sounds like you took a pretty rough session. I have to agree with Zengrifter, though. Evaluate your play, adjust your betting size if needed, and get right back on the table!
I have taken many BJ beatings like you described as well, 100 plus betting units. With the best counting methods and bet spreads imaginable, losses like this do happen. The most optomistic advantage counters have at the game is supposed to be around 1 1/2%, under the best of circumstances. However, that 1.5% advatage is the best possible outcome in the LONG RUN! The hard part is determining the number of hands and table time is required to arrive at the "long run." In my experience, my bankroll usually turns positive in about 2 days, (24 hours table time) durring the worst of down turns. I have heard from fellow counters of loosing streaks lasting a week or more. It can and does happen.
However, the good news is, as has already been posted, that the wave of Standard Deviation (Luck) can and does swing both ways. Providing you don't give up, you have many sessions ahead that will see you leaving the table with 100 plus betting units of the Casino's money (providing you have minimal heat), more often than you will leave it behind.
Theoretically, statistically, mathmatically, it makes no difference, but as a rule, I try to limit my looses at any one table to 25 units, or 100 units per Casino. This little idiosyntric rule works for me but really has no influence over results, just spreads out loses when I have a down turn.
Don't give up; counting does work!