Is it normal to have normal ultra sounds of my thyroid and a normal sestamibi but abnormal blood work?

Yes. Sestamibi scan should have been ordered if you had high serum calcium and high PTH and urine calcium is not low. It shows the abnormal parathyroid only 85 % of the time. The ultrasound is less likely to show an abnormal parathyroid and very dependent upon the person performing the ultrasound.
Yes. Endocrinologist can best decide what's happening not enough info for me to speculate.
Yes. An ultrasound checks for anatomical defect. The blood work checks for functional defect. You can have a normal looking thyroid (no nodules, normal anatomy) but can be hypo or hyperthyroid on blood work (abnormal function).