Cramps, backache, hot flashes! Help!

My husband and I are TTC baby #3. We have a 5.5 year old and 10.5 month old. I am still nursing our daughter so the symptoms I remember having from our previous two, I fell will be out weighed by the breastfeeding. I am on day 19 of my cycle and for the last 2 days I have had the most painful (next to labor) cramps ever. They were low (uterus area) and centralized. They radiated through to my back and caused the pain down my legs and into my knees. I have never felt anything like it before! Could this be implantation? What is it!? :-(