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I am sorry to hear that Bella is not feeling well. From your description, it sounds like she is experiencing abdominal pain. This is evident my the fact that she is reluctant to lay down. The fast heart beat makes me concerned that she is painful but may also signify that she needs fluids and has lost enough bodily fluid to be what we call hypovolemic. These things need to be evaluated by a veterinarian as I am concerned that she either has a severe gastroenteritis or a blockage from something she ate. Does she have a habit of eating things she shouldn't eat like toys, socks, clothing, underwear, trash or other non-food items?
The signs you are describing are not consistent with a dog in heat so I am worried that there is something else going on with her.
Also, if you are near an emergency vet clinic, it would be best to have her evaluated tonight, however if there is no emergency vet close to you then I recommend having her evaluated by your vet as soon as you can tomorrow. This is a case of the sooner the better.