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I would agree that it sounds like a heat cycle. This is a stressful time on the body. This is especially true with the first heat cycle. They will often pant and tremble and pace during this time. These are all normal responses to stress in dogs.
I would just let her have the heat cycle and then spay her. Spaying can be done when they are in heat, but I prefer to wait until after a heat cycle to make the surgery less bloody and easier.
I hope this helps, let me know if you have any other questions.