Welcome!
To your question - It differs from dog to dog. My girl Tysa never got sick, but she did get the runs during her first heat. Their bodies are going through so many things, that it's common for them to be a bit "off". Whatever is going on, give it a couple of days, and if there is no improvement, then see a vet. She will progress if it is due to her heat, but if it doesn't, then you have to worry it's possible something else is cause it.