No period for 4 weeks, neg pregnancy test?

I've not had a period for 4 weeks. 6 home pregnancy tests were negative and the doctors test was also negative. She poked and prodded my stomach a little and said it all seems okay. I'm 32 and have children so knew it would be negative - I think i'd have known if I was pregnant!

When I asked the doctor when I should be concerned over not having a period, she said to wait 6 months. SIX MONTHS!! I'm usually as regular as clock work.

Every 3 or 4 days i have some mild cramping that lasts a few hours, whichI kept thinking was my period starting but nothing.

Anybody experienced the same thing and have any ideas what it could be? Obviously as i've said, i've been to the doctor for a pregnancy test which was negative so it's not that. What else could it be?