Concerned about it,
You say it is on the top. Was that after you folded it?
The other question is what would make you fold your tongue in the first place? (I have a bad sense of humor but still had to ask.)
It could just be the blood vessel on that side of you tongue filling with blood that cannot drain while the tongue is folded. Usually there are two major vascular bundles in the tongue consisting of the artery, vein and nerve called the lingual vascular bundle. It may not appear on the other side due to the fact that we are not bilaterally symetrical. The bundle may be in a slightly different position on one side.
If you are worried about it get a dentist, oral surgeon or ENT to look at it. The cost of an evaluation is small in comparison to the stress and worry you are putting yourself through.
I hope this helps.
Cheers,
Mike