Advice

I just gave birth four months ago. My period returned like normal. But I haven't had one this month. My husband and I did have unprotected sex a couple of times. And twice he ejaculated in side of me. On Monday and Tuesday I had brown spotting, I've also had some mild cramping. I keep taking tests but they are negative. Is it possible that I am? When should I test. I didn't track my period like a dummy.