Hi An_244763,
I apologize for the delay in getting back to you.
Although I cannot diagnose on line I can tell you that most likely the pigmentation has actually been ther before now but you had not noticed it until now since you were actively looking at your tongue.
I have seen hundreds of patients with brown spots on the tongue. Most of the time it is normal racial pigmentation which occurs after a break in the skin, for example, a patient bit their tongue and when it healed the place they bit healed but looks darker than the adjacent "normal colored" tongue.
The darker your skin is the more melanin producing (pigment producing) cells you have. These cells can occur as often as one in three skin cells to one in twelve skin cell for fair skinned people. If there is trauma the melanin producing cells break and the melanin spills out into the tissue below the break in the skin. This is why people often have dark skin at an area of prior tear in tge skin and that tissue stays dark even after the skin heals. The official name for this is post inflammatory hyper-pigmentation. The reason a dark area remains is that the specific cells that "eat" the melanin are very very very slow.
You can certainly show your tongue to your dentist or physician at your next visit to get a clinical diagnosis. And if the appearance of the brown spots changes with respect to size color or texture you should see a doctor. I would recommend either an oral surgeon or a dermatologist familiar with the skin of the oral cavity.
You did not mention if your tongue is still causing you pain however if it is you can reply to this post and I will get back to you.
I hope this helps.
Dr. Brown