Feels like I'm going to start my period but...

It just won't start! I'm exclusively nursing my 3 month old son. He doesn't take a bottle or pacifier. He nurses every 2-3 hours during the day and every 4 at night. I'm cramping and bloated and have been feeling like my period is coming any minute for 2 weeks now. It's driving me crazy. Anyone else experience this? How long did it take you to start?