Chewing and biting indicates that Tina is uncomfortable and /or itchy in this area.
The most common causes of this in a dog would be flea allergy dermatitis, impacted anal glands, chronic diarrhoea or joint pain.
As she is 6, joint pain is less likely particularly if she has no lameness/stiffness.
Although she has been given Frontline, a flea reaction is certainly still possible. We have noticed that Frontline spot-on is not as effective as it once was and many fleas are now resistance. I would suggest we tackle this as a potential issue first and foremost. Ideally, we need to use a prescription strength flea treatment such as Advocate/Prinovox or Nexgard Spectra. At the same time, the house should be treated for parasites (hot wash bedding, vacuum, use a flea spray such as Indorex) and any other cats/dogs/rabbits must be treated.
Anal glands frequently become blocked, especially in smaller breeds. A bout of diarrhoea can lead to this. They need to be assessed by the vet and potentially emptied. Sometimes, dogs will even develop infections and abscesses which can make them unwell; the sooner these are addressed and treated the better. Often, antibiotics and anti-inflammatories will be prescribed.
The diarrhoea itself may be causing the discomfort as if her back end is sore, she may be attempting to alleviate the pain by chewing her rump.
Not eating and diarrhoea are a concern and while they may potentially be linked to her discomfort, there could be something else going on e.g. gastroenteritis, pancreatitis etc. We really need to look into this. If she is lethargic or dehydrated because of the lack of nutrition and diarrhoea she may even need to be supplemented with fluids, for example, with a drip. I would be keen to ensure she has energy and her gums are pink and wet . You can tempt her to eat with some bland and digestible warmed up foods such as chicken and rice. You can also make a 'broth' from the chicken water to offer her to drink. I would certainly be more concerned at the moment about her not eating and having diarrhoea than her back end. We need to address this first.