Hi there
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.
Please comment to let us know if this answers your question or if you need more information.
Health Nurse
For other readers, please feel free to leave a comment, or let us know if this was helpful.