My tummy is swollen and ineed to know why?

im 44 years old im 5ft.3ins. tall and have been under weight all my life(6st.7lbs.)then suddenly my weight has increased to9st.8ibs. i havnt put weight on anywhere else at all i look very heavily pregnant my waist has gone up to 36ins.my hips are30ins my bosoms are 38b.i had a hysterectomy in feb 95 so i know im not pregnant.its really worrying me i look horrible and its solid not soft but my doctor says its fat .is this a natural process at my age or should i insist on a second opinion?