What you have is medically called Median Rhomboid Glossitis. As you have noticed it recently, is red and sore it is most likely fungal infection
that is yeast infection). It looks smooth because the normal structures called papillae are missing there.
It is important to know if you smoke cigerette/beedies, you have pre-diabetes
or diabetes, any dental problems, whether you have any immune suppressed state. Because these can have bearing in your tongue condition.
If you smoke, stop it.
You may need an antifungal oral gel for application to tongue. If the condition does not improve with that you may need systemic antifungal therapy.
If sore mouth is bothering, you can take pain killer tablet
You need to see a Dentist
or an ENT specialist
to determine the exact cause of your tongue problem and to treat it. If you are scarred of tests just get medicines prescribed by a doctor.
But if your problem is not solved with medicines alone you should not be afraid of tests as your health is very important.