16 days late

My husband and I have been TTC for 6.5 months now. My period is always spot on and if I’m late it’s like 2-3 days late. I had a blood test Thursday which said it was negative at a 1.0. I’ve been having bad back pain and bad lower pelvic pain. My breast feel fuller too.My doctor said if I don’t get my period by this week she wants to send me for a trans vaginal ultrasound. Any idea of what’s going on?