For what reason does my skin welt up whenever I itch or just slightly itch?

Dermatographism. When the skin is scratched, a hive develops. Most cases happen for no known reason, although it can be seen after one gets an upper respiratory infection, a medication, or collagen vascular diseases. Again, most common it comes for no known reason. Treatment is simple--take antihistamine every day (i.e. Zyrtec, claritin, (loratadine) allegra). Most will resolve with this. Take for 3 months and try stopping.