Cringed.

Long story short, we just had our daughter back in July. My first, what I think wad a period, was August 2nd-8th. I only bled for about a week after birth. I haven't had a period since then. I'm on the pill and I haven't missed a single day. I've been extremely neausuous and I've been kind of crampy, so I took a test which produced a negative result. No faint line whatsoever. A few days pass and I thought I was getting my period. Severe lower back pain, hardcore cramps, even medium/heavy spotting. It's been a few days of this and I still haven't gotten a period. Should I be worried? Advice?