Just make sure she is comfortable, watch her stool and urine. Also, watch for vomiting and stomach tenderness. Sometimes they get a uterine infection after heat if it gets serious enough she will have to be spayed. A call to the vet is free, ask questions.
It could be related to hormonal changes. But I would say that if a dog shows signs of lethargy or unusual tiredness at any time, then it might be wise to get her checked with the vet. It isn't necessarily the case that a female should show any symptoms after being on heat.
A cold wet nose is thought of as being a classic sign of good health, which often is true. But it is also possible for a dog who is unwell to have a cold wet nose too. So that isn't always a perfect guideline.
Eating normally is a good sign. But being tired (lethargic) is not. Best to get her checked out.
Also, the male dog she plays with is 4 years old and has been neutered.