This will not be a problem. She will still need her regular dose on 2/2/09 to keep her on schedule or you can change it to 2/13/09 and keep on for the rest of the year on that schedule.
The heartworm medication kills off all the heartworm larvae that are in stage "L3" and "L4" of development that have accumulated over the past month. It only works that one day and then the dog won't end up with the adult heartworms in the heart because it kills the young ones before they get to that stage. As it takes more than 4 weeks for the heartworm to grow to an adult by worming every 4 weeks you keep killing off the babies so none live to grow up.
The poison just stays in the dog's system for a short time. So when you give a second dose like you did by mistake it would not do any harm same as it doesn't when you gave it the first of the month.
Hope this helps you!
Pet Health Care, Rescue,Train,Breed
30+yrs pet vet care & nursing, rescue, behavior & training, responsible breeding, small animal care