Hcg levels not rising properly

Hi, so my first test which was 13dpo was 16, second test at 17dpo was 91, 3rd test at 21dpo was 570 and my latest at 25dpo was 1100... my doc doesn't know why it's not rising properly and I'm scared it's ectopic because of my history (miscarriage, endometriosis and family history of pregnancy troubles). Last week I had severe lower back pain with cramping. Had an ultrasound done and saw nothing. Have another ultrasound booked for next week with hopes something is seen. Has anyone else had a similar issue and if so what was it?