I Have Had Pain In My Pelvic Area (left Side)

I have had pain in my pelvic area (left side) for about a month now. I went to the doctor and they did a pelvic ultrasound and said there was just some fluid surrounding my left ovary, and that the fluid will go away. I am concerned because there is a lot of pressure and it has been so long. Should I be worried, and would fluid cause this much pain?