That definitely looks like overfeeding to me. But it could also be an internal parasite, or even dropsy.
Do a 50% water change today, because if she is being over fed, there is a good chance her water perameters are off. I definitely don't think this girl is preggers as she isnt exhibiting an egg spot. I would also change her water again with a 50% change tomorrow and perhaps the next day as well. Then I would do 25% water changes every day until the problem fixes itself.
Feed a very tiny piece of blanched frozen pea and then stop her feed until at least 4 days from now to see if it's just constipation. Also please make sure if you are feeding pellets to soak them first in the future!
If that doesnt fix it you may be looking at an internal parasite, sorry to be icky but is her poo normal or stringy? Is she pooing at all? I am always hesitant to medicate until after trying water changes. Many problems do get better with simple water changes and taking them off their feed for a few days. Best medication for intestinal parasites is Jungles Parasite Clear.
I have a bad feeling though, that this is dropsy. Mostly because I see a small patch of white behind her gill, which looks like ich. Ich and dropsy sometimes come together in new Bettas that have been too long in those icky little store cups without a water change. Is she swimming normally? Is she rubbing against substrate or any decorations or filter intakes? There is no known cure for dropsy that has been proven, it is best to just do lots of water changes while waiting to see if it's constipation. If she doesnt get better within 3-4 days I would pick up some Jungle's Parasite Clear.
I hope your little girl feels better soon and feel free to PM if you have any other questions.