I need some help figuring this out. Please help!

The answer may be right in my face but I can't figure this out for the life of me. So I can't figure out when I got pregnant. Am I 5weeks 4 days or am I 4 weeks 3 days. Now yo explain this. I went to the hospital for a possible miscarriage or etopic pregnancy yesterday. I was only spotting and had pressure. They done blood and urine but no ultrasound. My hcg levels came back at 100 the doctor said there's really no reason to do a ultrasound just yet because my numbers aren't high enough to see anything and that I could just be earlier than I thought. I go to the doctor tomorrow for a redraw but I won't know the results until Monday. So now iv been sitting here trying to figure out if i ovulated a week later. If so it would've been the 6th of this month. But I found out on the 16th I was pregnant. My period came on the 15th of December and November. Normally my cycle was 28 days not 30. I should've ovulated on the 29th going by my period and I did have sex on the 29th too. So my question is can I get a positive test 10 days after or is it more likely from the 29th?

Also does 100 seem low? I think it does