Yes, that's exactly it. The cancer is still growing, but apparently hasn't yet done enough damage to slow her down, but it certainly will happen at some point since your mom has discontinued treatment.
I'm so sorry that HIPPA laws prevent your mom's doctors from speaking to you about her condition. You might consider making an appointment with our own doctor and discuss the physiological response of cancer with him. Our family doc was so good with us during my dad's illness in that regard - but that was pre-HIPPA.
On the contrary, she actually PUT ON a little weight. She has no jaundice, but has uncomfortableness in her abdomen, sometimes painful, but not off the charts. So do I understand you correctly in that she could look and sound fine one day, then this creep up on her rather quickly?
I had the same questions when my father died of gallbladder cancer years ago, and never did get a good answer from his doctors. For that, I had to turn to our family doctor of 20 years.
It all depends on what organ the cancer metastacizes to, and there's no way of predicting that. It may wrap around and block a section of intestine causing a full obstruction with high fever and vomiting. It could go further into the liver and pancreas and you'll see severe jaundice, diabetes, and of course - tremendous pain. No doubt you've already noticed weight loss. My dad didn't have too much pain until his last month when the cancer hit his liver, pancreas and probably his spleen. As you already suspect, it will probably happen out of the blue, and very quickly.
It's horrible to have to watch a loved one slowly deteriorate from a terminal illness; and even worse to not get a straight answer from the doctors. I think the docs shy away from death almost worse than the patients and family members - as if it's an insult to their practice. My dad made the decision to stop the chemo as it wasn't making a darn bit of difference - didn't even slow the progression - so he opted for as much quality of life as he had left to him. It wasn't easy for the rest of us to accept, but we had no other choice.
If you haven't already done so, consider posting this information on the Cancer forum. God Bless.