My 2nd beta came in today. 2 days ago it was 4 (at 13dpo)---worryingly low. Today it's 14 (15dpo). More than tripled in 48 hrs. Still too low, but growing nicely. Retest Monday to ensure it's still at least doubling.
I'm choosing not to worry. For now, it's growing at a great rate and the line is getting darker.
I am highly impressed that I had tests picking up such low values though! That's kind of impressive! Last night a digital came pack positive and I couldn't have been more than 10hcg.