I am 5'7 and weigh 115 pounds. Sounds thin, right? WRONG. HELP!

well my legs are thin and everything else is but my BELLY. I hate it! it protrudes from my body when I stand up and when I sit down I get serious gross rolls. and when I stand up I have red lines from where the rolls were. its so fatty and gross I hate it. but when I calculate my bmi it says I'm underweight. WHAT?!
I am 5'7 and weigh 115 pounds. Sounds thin, right? WRONG. HELP!
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