Thank you for your patience. These symptoms are very interesting in your girl and you are going to have to continue to keep a close eye on her if you can. While it isn't too uncommon for dogs in heat to become slightly lethargic and go off food a little, it sounds like she is really quite lethargic today. Other potential causes for her behavior that we need to consider includes everything from a womb infection (pyometra) although typically this would occur several weeks after a heat, early gastroenteritis, a toxicity or even an internal organ issue.
Given she is young and in heat, these symptoms are all likely to do with the heat rather than anything else. To be safe though, please keep an eye on her mucus membranes, capillary refill time and respiratory rate tonight as follows (do be careful that your girl doesn't try to bite you):
Mucus membranes - flip her lip and look at the color of her gums. They should maintain a nice salmon pink color. Get her to the emergency Vet if they appear white or very pale pink, or if they are a dark deep red color.
Capillary Refill time - this measures blood perfusion and test this by putting your thumb on her gum to apply pressure. After you release your thumb you will see the gum blanch. Capillary refill time is the amount of time it takes (in seconds) for the gum to return to a healthy pink color from the blanched white color. If 2 seconds or less don't worry - if it is taking significantly more time, again - off to the emergency Vet.
Respiratory Rate - if she seems to be panting or breathing rapidly throughout the night, this is a sign of shock and or pain and a signal for a trip to the emergency Vet.
If she continues to be very lethargic through the day and turns her nose up at her food again, then it would be best to play it safe and have her checked over today if possible. I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
PS: If you have additional questions after you rate the question, you are welcome to request me for additional conversations if I am on-line or by beginning your question "Dr. E..." or "Pet-doc..." and others will leave the questions for me.