According to worldwidewords.org, "dot and carry one" (as used in the book) had implications that the heart skipped a beat. This would coincide with Barrie's answer about what the true meaning is. Referencing a Captain Francis Grose in his Dictionary of the Vulgar Tongue from 1785, the site also offers an alternate explanation:
(Grose also mentions hopping-Giles as another slang term of the time for a person with a limp [...])
The notes provided at the end of an online version of the book seems to have come to the same conclusion defining the meaning as,
An irregular "thump, thump."
(An irregular pulse / heartbeat equates to a skipped beat. In other words, the character could have just said he/she felt heart palpitations.)