What Causes Cramps In Body In A Nursing Mother?

I had my child three months ago and have been healing well. The last three days I have had significant cramp-like symptoms on my lower right side that feels similar to the cramp you get after a strenuous exercise. It is sore from right below my rib all the way down to my belly button on the right side. This pain has lasted three days. Any idea what it is? I am breastfeeding and my period has not yet returned.