I've recently started getting really itchy all over my body but the worse spot is my stomach, back and my thighs. There are no noticable bumps or rashes but when I do end up itching them I get some pretty extreme welts. What could possibly be causing this

Dermatographism. Your condition sounds like dermatographism. This is a type of urticaria (hives). It normally resolves in a few months, but some patients may persists with symptoms for years. Treatment with anti-histamines like zyrtec or Allegra (fexofenadine) are usually very effective. Be sure to consult with your dermatologist to confirm your diagnosis and evaluate you for possible triggers.
Urticaria/Hives. It sounds likely you have some trouble with hives (medically called urticaria) and the hunt for a specific cause is often illusive. They are often intermittent and when they occur, can be quite itchy and distressing to the affected. Make sure you keep cool, moturize skin, and take some otc zyrtec or Benadryl (diphenhydramine) regularly for a couple of weeks. If not better, consult your doc. Good luck.