Read and understood that you had unprotected sex a week ago and the classical burning as you explained started 4 days ago.
This is due to urethritis, most probably due to trichomoniasis.
I would advise you to get urine tested and get appropriate treatment from your Doctor.
Take plenty of oral fluids.
Take care with condoms.
Get your partner treated as well.
No sex till you are alright.
Hope this helps you.
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