Did this come on suddenly? Or have you noticed a gradual tendency to lethargy? Sudden lethargy often indicates a bacterial infection, a worm load that has reached the tipping point (clogging the intestines so nothing moves through), or a stuck egg. Obviously, you need to examine her carefully, and consider her movements over the past few days to determine if she's been exposed to something contaminated.
There are two kinds of abdominal swelling. One is caused by liver disease and causes swelling between the legs and just in front of them. It's called ascites and is caused by fluid retention in the outer tissues of the abdomen.
The other kind of swelling is located behind the legs and under the vent. It's often caused by a reproductive infection or a blockage, usually by a stuck egg.
Tell us where the swelling is.
Also tell us when she laid the last egg and if she's had shell quality issues such as shell-less or thin-shell eggs.
Tell us if she likes to dig in soggy, smelly, moldy compost. She could have picked up a nasty bacteria.
Yes, Safeguard for goats is a worming med you can get over the counter to use for chickens. Fecal float is a test a vet can do on a poop sample to tell if worms are present. It's simple, quick, and cheap.