VFT's really do best as an outdoor plant. They really need the direct sunshine, as well as American pitcher plants, more so than many other CP's. Like American Pitcher plants, they require a dormancy period in order to be at their best the following year, including propogation. Often times a VFT that hasn't has adequate light AND winter rest, will die, in the process of flowering. The energy to produce flowers and keep going is great and it is like "the last straw that broke the camel's back".

In their natural habitat, they are obviously outside, but they also expereince a dormancy period. If I had to draw a pie chart / percentages as to the variable that go into dormancy and their importance, I would say that reduced photoperiod would be ~70%, followed by near freezing temps at ~20%, and then less food and water comprising the rest.

A VFT dying after flowering, from lack of light and winter rest is one deleterious effect and another is the general decline in health. I think it is that decline in health is what you are experiencing.