What Could The Canker Sore On Vagina Be Indicative Of?

Over the last couple of days I have gotten a bad canker sore on the opening of my vagina. I hve two other smaller ones and they seem to be getting smaller and less painful. But the one on the opening of my vagina still hurts when I urinate. I haven t gotten tested for and std in about 8 months and have had the same sex partner since then. I have never had theses sores before and I have gotten checked multiple times about a year ago and the tests all came back negative. I just need some feed back because I m really worried.