Negative pregnancy tests but my period is late

I generally get my period every month, I've been forgetting to take my pill a lot. But when I do a pregnancy test it's negative.

So i assume I'm definitely not pregnant because the tests negative and I'm 8 days late. But it doesn't explain why it felt like I was getting period symptoms, cramps and back pain. It felt like I was going to get my period, that I was even wearing a pad just in case.

But my pregnancy tests are negative