Women normally release one egg each month. The egg originates from one of the ovaries, and will then travel down through the fallopian tube and eventually reach the uterus. In the case that the egg was not fertilized that month, it will be reabsorbed by the body and simply disappear. Ovulation pain can be caused by blood released from the relevant ovary, or slight contractions in the fallopian tube while the egg journeys downward. If you are a regular "sufferer" of ovulation pain, you will feel ovulation pain on one side only because only one ovary either the left or the right one will release an egg. These sides have a definite tendency to alternate. That means it is logical to feel ovulation pain on one side one month, and the other the next month.
Ovulation pain should last for a few hours. It can also be accompanied by a myriad of other symptoms that indicate you are fertile. For instance, women who are charting to conceive (or avoid!) know that ovulation comes with a sudden surge in temperature. Those who check their cervical mucus know that it is normally thin and egg white-like during ovulation, and many women have tender breasts, are a bit tired, or have ovulation spotting.