I'm 16 weeks pregnant and the skin on left side of my stomach and back is sensitive and extremely painful when touched. but i have no rash. why?

Pregnancy Pain: You are in the 2nd trimester of your pregnancy. You are only 25 yo, although, I do not know, whether this is your first pregnancy. Most probably, your pain is the result of compression of the nerve roots emerging from thoracic vertebrae. Increase abdominal size, might be putting pressure on the back, resulting in pain in the areas of your stomach supplied by the compressed nerves. See your obgyn.

Possible shingles: From the tone of your question, i get the impression that you are thinking about shingles. Sometimes shingles can present without a rash and sometimes pain will appear before the rash. Discuss your concerns with your ob.