Muscle cramping is generally a cause of inadequate electrolytes / hydration or simply our body's way of shutting things down in an act of self preservation. It can also be caused by an inadequte recovery time between workout sessions which will bring on an earlier onset of muscle cramping than normal.
If you have just increased your volume of exercise or are new to the TRX, muscle cramps are not uncommon. The suspended lunge you describe also poses some significant demands on the entire leg, not just to do the movement but in maintaining a strong single leg stance.
Make sure that you are adhering to good nutritional principles and that you are well hydrated prior to working out and give yourself adequate recovery times between workout sessions. The advice you got from the TTC trainers was good. If you get a cramp, pause and try to stretch things out before resuming but if it returns you need to move on to exercises that target other areas of the body or stop your session all together. Your body is telling you it has had enough. Be sure to focus on your post workout recovery nutrition and get lots of rest prior to your next hard strength session. Performing some low level recovery cardiovascular exercise will also aid in speeding your regeneration.
I hope that this helps