hcg levels not doubling and are low

Haylee
Hi everyone my doctor doesn't seem to confident in my pregnancy. Because my hcg levels are rising but not doubling my 1st child I didn't know I was pregnant till I was 5 months so being early is really driving me nuts already. And this is really got me worried if anybody could show me some light or advice or tell me what might happen that would be great . 
Last Wednesday my hcg was 61
Friday it was 91
Sunday it was 124 
I have to go in every 2 days and she won't do an ultrasound cuz she said I won't see anything till my levels get to 2000. :(