Help - quite low betas with a slow doubling?

Small Town Girl

Finally got a bfp again (2 previous Mc) and was over the moon, even if the line was light!

Went for beta Monday and it was 12 (ok MAYBE I ovulated or implanted late?)

Beta again today after 48 hours: 18mIU. So only a rise of 50%, Or an approx doubling time of 82-84 hours...

I’m so scared for a chemical or early Mc or Ectopic :(

Any stories or experiences ?! Help :(