She may have vomited up worms that died due to the heartworm med. As roundworm life cycle is about two weeks you may want to worm her in two weeks.
You can bring a fresh stool sample to your vet to make sure round worms are the only parasite she is dealing with.
If they are then you may want to dose her a couple of times middle of the month between heartworm med doses.
You can read up on worms here
http://www.thepetcenter.com/exa/worms.html
Hope this helps you!
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