20 had my first baby...... nd advice

i am 20 years old i have kinda always been over weight but i have carried it well, until i had my first child at 18, i cannot seem to loose the weight and now i am havin severe back pains no one seems to know exactly what is wrong with my back.. i am wondering if it because i have not been THIS overweight before or what... i have a small bone structure i amd5'4'' and i weigh 220 pounds  sence birth i have not lost any weight nor have i gaind....