Hcg rising but not doubling

I got a very early positive at 3 weeks 3 days. Dr tested HCG at 3 weeks 5 days and HCG was 86 (last tues). At 4 weeks it was 169 (last week Thurs). I had some brown spotting with one steak of pink over the weekend and so my dr rechecked my HCG Monday. It was only 279 and she wants to recheck in two days. I had a very long miscarriage process from Feb-may so I was so excited for a rainbow baby but now I'm just sad. She says we should still be cautiously optimistic but I'm having a hard time. Anyone have encouragement for me??