If its a tiny white speck, like a grain of salt, located under the female’s belly, between her ventrals and her anal fin. The spot is none other than the ovipositor tube, which the female drops during spawning, and out of which the eggs will come out
If the spot is only on one side, looks fuzzy or pussy (yucky) then it may be a fungus or bacterial infection (or both). It it starts to grow in size or have cotton-like extensions - treat her right away with a broad-spectrum medicine, like Jungle Anti-Fungus.
She sounds stressed. Is she by herself? She may be hiding from another agressive fish, usually another betta female chasing her till she sits still. My alpha (Reena) did this to Shelby and Ruby all the time before they were divided up. The two girls would just sit at the bottom of the tank, not daring to move or Reena would chase them again.
If that is the case, you can make a divider for your tank. I use a sheet of acrylic (3mm plexiglass) I purchased from Home Hardware in a small sheet. I cut it to fit using a jig-saw with a metal blade on a slow setting, then drilled a bunch of 3/16" holes in it using a variable drill to let the water through.
Now, all my females are happy, moving around, well fed and comfortable.
Keep us posted - what's her name?