How old and large is she? Has she ever been with a male?
I actually don't know if it's swollen or not. The answers to the questions may help those that might be able to help @Yvonne G
It looks swollen like after some mechanical trauma. Has she mated? Or maybe deposited clutch of eggs today or yesterday? Or... Maybe she passed a very large hard stool? Or bladder stone? Or foreign object she ate? I hope @Yvonne G checks in soon with advise. Meanwhile give her nice long soak, can put some mineral oil on her vent and gently rub it in with gloved finger. While doing so you can feel if there is anything hard inside. Is she eating/pee/pooping/acting normal otherwise?
I agree it looks like she passed something that caused some trauma...like an impaction, a stone, maybe she was constipated and had a hard poop. When was the last time she pooped?