This is Dr.Christian. Welcome to JustAnswer. A few questions will give me a clearer picture of what's going on.
She's spayed, there wasn't any blood, and she has been losing weight lately (over the last 6 months or so) but she hasn't shown any urination issues except not being able to make it to the doggie door in time. She has started actually sitting down instead of just squatting when she does her business though.
Thank you for the information. The squatting may be due to joint pain and arthritis at her age. The mucous and painful urination (producing a small amount of urine at a time) is typical of infection. I'm concerned that the accidents she is having is due to an underlying condition that is also causing the weight loss at her age. Here are some possibilities:
All of these conditions cause a dog to produce more urine which can be a cause of accidents. Of course, arthritis and joint pain could also may it difficult for her to reach the doggie door. Many of these conditions will also predispose a dog to develop urinary tract infections. In this case it would be best to have her checked by your vet. A urinalysis and possibly bloodwork may be recommended. If you have more questions please let me know.