first take the angelfish and isolate her in a separate tank unless she is now the only one left in the main tank
sometimes a fish swimming erratically or upside down means it has a swim bladder disease. Let me ask you, so I can be certain it is, does it ever lie on its side at the bottom of the tank?
If not, does the fish have any white spots on it?
But I bet it has a swim bladder problem. Swim bladder problems are not life threatening unless you do not treat it. You need to quaratine the fish. There is probably a blockage in the intestines. Sometimes fish take in too much air while eating which causes the fish the swim erratically.
First isolate the fish in a hospital tank (regardless if the fish has a swim bladder problem or not). Next you want to feed it roughage like peas; but before you do this, do not feed the fish for 3-4 days so the digestive system can be cleaned out.
After 3-4 days of fasting, feed the fish peas.
Also while the fish is fasting, raise the temperature to about 80 in case it gets a secondary infection (bacteria), at high temperatures, bacteria does not do very well and will die off. Also cover the hospital tank with a dark towel, bacteria does not do very well in the dark. Also you will want to add some salt which also kills off any secondary infection that may develop. Also add Maracyn TC to treat the secondary infection.
Continue to feed it peas as its sole diet.
Usually swim bladder disease occurs when the fish are overfeed especially pellet foods.
If it is not a swim bladder disease, then I would still remove the fish and treat it with Maracyn TC (an antibiotic).
hope this helps