Symptoms

Okay so I haven't had a period since March, I got off bc back in September and my period was regular until I missed my period in April and May. I've taking test and they have all been negative. I called my doctor and she said its normal for my period to go back to being irregular since I'm not on the pill anymore. I've been having some cramping every now and then for about a month now but no period, and just the other night when I went to bed (I sleep on my stomach) I was having some lower back pain. Anyone know what it could be?