Low HCG levels

So at my first appointment they said my hcg levels were 108 and couldn’t see anything on the ultrasound, they claimed i was just really early. i was thinking i was around 4 weeks. went back 2 days later and they were 290. almost tripled but the nurses have acted like it’s normal but not normal at the same time if that makes any sense. im very worried because i thought that was good that it raised like it should but at most 4 week ultrasounds you can at least see something can’t you? any had this problem? lower levels then expected and just been earlier then expected? everything turn out okay? i’ve had a previous miscarriage and my brain is fried from not knowing what to think or do. also btw i track my periods each month so the first day of my last cycle was march 21st. which would now make me almost 6 weeks exactly. how does that work? they always base it off your last period but they are saying i’m only actually 3-4 as of now. what i thought i should’ve been 2 weeks ago.