I apologize for not responding sooner.....for some reason I was not notified that you had replied.
Many times when you see this type of change in a cat's coat at this age it is from a problem called hyperthyroidism. This can cause major changes in coat condition and cause excessive weight loss or gain - depending on the cat. What I would suggest first is taking your girl into the vet and asking them to check thyroid function and also for diabetes since this is done from one blood test. To get this horrible mat off of her, I would use a simple little seam ripper and gradually cut thru it. You can get them at craft or fabric stores. Just be careful not to poke her with the pointy end, but there is much less chance of cutting her with this than with scissors and it is not as painful as using clippers.
I apologize again for not responding!!!! Please let me know if I can help in any other way!
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Lori McLaughlin
Your veterinarian should ALWAYS be consulted first and foremost when your pet's welfare is at stake.