Hello and thank you for your question.
Can you tell me if this appears to itch?
Oh your poor girl,
First the question regarding grooming. Regular grooming should not cause skin problems, however I would not recommend bathing more then every other week as this can dry out the skin.
If she got wet and the skin underneath did not dry due to the matting that combined with the matting can cause skin irritation.
You can try washing the area 2 times a day with a mixture of antibacterial soap with a small amount of 3% hydrogen peroxide mixed in. Pat the area dry do not rub and apply an antibiotic ointment such as neosporin.
Sterile eye wash such as the ones used to clean contacts can be used to rinse the eyes.
Benadryl may be helpful for the itching here is a link to the approved over the counter medications approved for pets and their recommended dosage.
If the rash or the eyes do not appear to be getting better I would recommend a trip to the vet to rule out any infection.
I hope this helps, I too am a shih-tzu mom and they are the best. Please feel free to reply back should you need further assistance.