The symptoms you reported (feeling of pressure right under navel when urinating) are not specific enough for a definitive diagnosis to be given.
Cystitis (inflammation of the bladder) might be causing the feeling of pressure. However, to have only this symptom is very rare. Actually, other common symptoms of cystitis (frequent, painful urination, discharge from the outer urethral opening) are usually present, too. Therefore, it is highly advised to visit a gynecologist or urologist for further examinations (urine deep-stick test, urine culture, mictional cystography, ultrasound examination of the bladder). The treatment would depend upon the diagnosis.