Hi, I can understand your condition with sharp pains in belly button area, nausea, headache and difficulty in urination. All of these symptoms are most often seen in some urinary tract infection or a stone in urinary tract. The second important cause can be in Pelvic inflammatory disease. You have not mentioned any history of discharge per vaginum so the possibility of Sexually transmitted disease is less likely. I suggest you to meet a surgeon for ruling out ureteric stone or kidney stone which might be causing the problem rather than thinking about Sexually Transmitted Diseases. Take care.