On The Left Top Side Of My Outer Lips In

On the left top side of my outer lips in my private parts, I have a quicky, pulsating pain that occurs when I am sitting in an upright position. It is not all the time, but I am feeling it more and more each day. This has been an on again/off again pain for about a year. It is always on my left side. I can't find anything to tell me what this is. Can you help me?