Pregnant with 4mm endometrial lining?

Hello everyone. Hopefully I’m in the right group. I found out I was pregnant last Monday when I went for my pre op appt for my tubal ligation that was scheduled for that Wednesday. Based on my suspected ovulation date I would have been somewhere around 3 weeks 5 days or so. I had an ultrasound the very next day for unrelated reasons and they saw on that ultrasound that my uterine lining was only 4mm as well as a complex cyst on the ovary that I ovulated from. So far my HCG is rising appropriately and everything seems to be okay. No bleeding. I’m 5 weeks pregnant today. My doctor is very concerned this could be an ectopic pregnancy. So, I’m curious, those of you who have had an ectopic pregnancy - does this sound like what happened for you? No symptoms and suddenly you had a lot of symptoms of ectopic? Has anyone in here conceived with only a 4mm endometrium? I’m just really confused. Could I not actually be pregnant and maybe the cyst is secreting HCG?

I am aware of the septate uterus and was already aware I had it. So not concerned about that at all.