What Causes Lower Abdominal Cramping?

I've had mild cramping in my lower abdomen for two days and earlier while driving for a long period of time it began radiating into my upper/inner thighs. I've also been experiencing some bloating mild mood swings and have had mild cravings for food I hate. I've taken a home pregnancy test but the results were negative. Is it possible to experience these symptoms within days of concieving or is it possible pms symptoms?