What do you mean when you say your ovary is swollen? Your ovary is pretty deep inside your body, so in most cases you don't feel a specific lump on the surface of your body if you have a cyst or swelling there (the cyst would have to be enormous to cause a localized lump on the outside of your body and it would be unlikely that would develop overnight). Sometimes people with cysts or ovarian growths have more general swelling/bloating of their pelvis/abdomen, though.
If you have a lump on your lower left side it might be digestive issue or lots of other things--but a doctor could tell you more!
I recently had a benign dermoid cyst removed on the right ovary and it was the size of a tennis ball and it could not be felt through the skin. (i'm a size 4 if that helps) There are many things that it could be. I wouldn't ignore it though. I had lower back pain that would not go away and I ignored it until I couldn't take it any longer and they thought it was kidney stones until the CAT scan came back. I'd see your doctor just to ease your mind