Some women, during ovulation, get the sensation of pressure, dull pain or actually strong pangs of pain. There are two common theories regarding this sensation:
The first is when LH reaches the ovary, in order to create ovulation. This means removing the egg from her follicle so it can get to the fallopian tube. This causes the tiny muscles around the ovary to contract in order to move the egg. You might then feel a pain that comes and goes, of a pulsating-like pressure.
The other common reason that usually refers to a different kind of sensation where the follicles in the ovary that grow significantly larger toward ovulation, take up more space and cause pressure that might be painful. That pain will be more static.
The important thing to remember is that LH appears about 24 hours before ovulation. But you are fertile before LH appears. Keep in mind that sperm can live in your body up to 5 days, waiting for the egg to show up.