I can understand your concern.
The Mantoux or the TB skin test is mainly done to ascertain whether you were exposed to or infected by the TB bacteria. The infection may be anywhere in the body, the reaction of the body can be seen on the skin area where the Mantoux test
solution is injected. It does not mean you have skin TB.
Since your chest Xray was clear, I assume you had TB of some other organ (TB can affect almost any organ in the body). This is called extra-pulmonary TB.
In some countries where TB is not endemic i.e. commonly found, treatment is given based on the Mantoux test. In countries where TB is commonly found, many people may test positive for Mantoux since they may be exposed to the bacteria.
Most of the times, if immunity is good, the TB bacteria cause a small infection of the lungs or elsewhere which gets controlled by the body. The bacteria then sometimes continue to lie there dormant, causing what is caled latent TB. Again, latent TB is sometimes treated, sometimes not, according to the local guideines.
I assume you had a condition which tested negative for all the common causes in your place, hence the TB skin test was done as a last measure. Since it was positive, it was assumed that this TB infection was the cause of your current symptoms and hence was treated. With treatmnet, you will test negative for TB bacteria.
Hope this helps. Wishing you good health.