Alana,
Without actually knowing the exercise that you had difficulty with, I'll make the assumption that it was possibly the single leg squat, or single leg balance lunge. While 20 extra lbs can definitely contribute to knee pain, more than likely it was your strength level and form that did you in. There is also the possibility of arthritis, or some other medical issue, which I can not diagnose. That would best be left to your physician, who I would also recommend you see before beginning a formal exercise program. Afterwhich, I would start with the balance squat(two feet firmly planted on the ground), the suspended hamstring curl, and suspended hip abduction(* note: if you have cartilage issues in your knees, the suspended position might give you some trouble). Be sure on the squat to only go through a range of motion that you can physically handle. If that means starting with a half squat, then so be it. That's one of the great things about the TRX, you can easily adjust for varying levels of ability. Once you have built sufficient strength in your legs, you can progress back to the single leg squat and balance lunge, but always watch your depth(range of motion) and match it to your ability level. I hope this helps.
Jeff Boyer, CSCS