There are some medical conditions and medications that can give you a metallic taste in your mouth. However, having a metallic taste in your mouth, and changes in the colour of your tongue, are not common symptoms of herpes or any other STI..
It was a good idea to get checked for STIs, since you noticed the symptoms shortly after a sexual contact. Given that all the tests were negative, the next step would be to see a doctor.
The doctor can do a complete history and figure out what may be causing your symptoms.
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