What Causes Painful Umbilical Hernia?

I have an umbilical hernia and it's always extremely painful around my period and randomly about two days of the month. I started charting my cycles to plan for a second baby and now know that the worst hernia pain occurs the day before and the day of ovulation. I've had a few doctors mention endometriosis in the hernia. Is this the cause, will this have a negative effect on my second pregnancy? OB says I have no contraindications for pregnancy.