No period. Advice please!!

I had my first period at 4 weeks postpartum. I am now 10 weeks postpartum and still have not had a second period. I'm on the mini pill and  I've had sex once. I've taken two pregnancy tests, both negative. Period was due 12/8. I took one text 12/10 and the other 12/17. I only pump twice a day because my baby mostly drinks formula(pediatrician orders). Could I still be pregnant? I am so worried! My baby was a preemie and I am so afraid of going through Taft again! Advice please!