It is unlikely that she is gravid so soon. Female ball pythons usually need a considerable seasonal warm up period before ovulating and then becoming gravid. If she did "pig out" in november, the mass near her tail could be a large urate or fecal matter.

Also, are you sure she's a female? It's not uncommon for male ball pythons to "lock up" when seasonal temperature begin falling.

Since you house your ball pythons together there's really no way to tell with any certainty when her last defication was and therefore she might be having a problem passing stool.

Keeping multiple ball pythons in a single enclosure is not recommended for reasons just like that.

It could be that all she needs is a soak in tepid water or possibly a trip to the vet to verify that the mass you are feeling is not an obstruction.

Hope this helps.

-adam