@Samd123 As she recently molted, her problems could be one of several things. First, she could have had a molting problem that was not obviously and has slowly been affecting her. Sadly if it is that there is nothing that can be done, as it tends to be a internal issue or something with their mouth parts.
The other possibility is being winter time the humidity is super low, and depending on the amount the heater is used, RH (room humidity %) can bottom out to 10%, and most species it needs 50% or better. If you have been misting once a day though that should not be the issue.
Another potential problem could be she is not eating enough. Ideally mantids are fed every other day, the amount varies depending on their food intake and her abdomen size. Me telling you to give two feeders every two days is not a good advice as feeder size varies, and so does the amount mantids will eat of their feeders (not to mention gender, or individual growth). That is her abdomen should look slightly plump at all times, never flat (starving), and adjust feeder amount to keep her well fed.
The last issue it could be is a bacterial infection. This can be caused from excess humidity (mold growth inside the habitat is a sign of a problem) with frass, feeder remains, and such leading a bacteria bloom. Or from sick feeders, typically feeders kept in dirty conditions and not properly cared for - such as crickets straight from a pet store.
A photo of your girl showing her filling the picture, including her abdomen would help possibly identify the problem, and will rule out some of the issues. There just is not much to go on without a photo, or not knowing much more details.
There is however no worry of her overeating as a nymph, as it tends to only be a problem for adult females that are fed
way too much. By too much I am referring to a prey amount for a mantid that it should eat in a entire week or even two weeks, but is fed that amount every few days for a period of time.