Puppy diarrhea after heartworm and flea meds: Did I double dose?

You did not mess up, its ok! They are both for fleas but work in different ways. Nexgard will kill the adult fleas (as well as ticks). However adult fleas are only a small part of the problem when it comes to fleas, because the eggs that may have been laid by the adult fleas will sprinkle off your dog, hatch in your carpet (eww I know) and the baby larvae will find their way back onto your dog (or other pets, or even you in very severe infestations). Sentinel renders the eggs laid by fleas unable to hatch.
This article below may help explain this.

Its possible that the medications may have contributed to her diarrhea today. If she is still eating, is playful and acting normally, see how she is in the morning, and let your vet know she had some diarrhea. Your vet may advise against giving them at the same time if he or she suspects that may have contributed. But the sort answer here it, you did not mess up. If the diarrhea is continuing to be severe, if she's vomiting, or acting dumpy at all, please see a vet tonight.

https://www.petcoach.co/article/how-to-prevent-a-flea-and-tick-infestation/

Thanks for asking PetCoach! I hope that helps!