Hard lump on left side of vagina

I have a hard lump on the left side of my vagina, the outer labia i think. It's been there about a week. At first I wasn't too worried, because I had the same thing a few months ago and it went away after a few days. But this one isn't going away. The last one was in the same spot too. But now i'm not so sure it was the same thing. Anyway, it's maybe a little larger than the size of a pea. At first it would hurt when i touched it, but now it only hurts if I apply a lot of pressure to it. It feels like it's getting harder but i'm not sure. It is under the skin, so i really don't know what it looks like. It doesn't have a "head" that i can see and it doesn't move around under the skin. I haven't noticed much color change. At first it was kinda red, and sometimes it looks more purple, but usually it is skin color. I keep thinking that it could be cancer, since it is hard. However i keep reading that it could be a pimple, but it doesn't look like a pimple. Should I be worried? Please help.