What is happening to my stomach?!

I am a girl turning 16 in just a week from now. Yet only thing I have noticed is my mid section. All my life I have been considered skinny to other people, but I have always had a bit of a tummy. Lately I have noticed it is a lot larger than it was on Wednesday. That was was when I have had an eager appetite and I have been eating not so much but as much as I would at a holiday dinner. Today was the first time in my entire life I looked down and could not see my toes. I am in huge trouble now. I am 5 feet 4 inches tall with a weight of 124.4(I used to be 118). That mean I have a BMI of 21.4 which means I am healthy.measured my waist and hips (this time letting my stomach out to show my natural measure) coming out with a 33 inch waist and a 35 1/2 hip measurement. My ratio came out to be 0.94 which is bad. Apparently this means I have a high risk of developing diabetes and heart disease. yes I know I am still going through puberty, but my period ended last week. My older sister that is just a year older than me and who has about the same weight as I do has a ratio of 0.78 which means she is healthy. I don't know what to think. I also started to realize have I started to get chubby cheeks. I am not allergic to anything, this is probably due to the weight I have gained. My stomach hurts every now and then and I can also feel my self get bigger there. My belly has gotten pretty full now. I feel as if I have been blown up with a gallon of water. My stomach will not stop making strange sounds as if I am going to burst. The pink shirt I am wearing now that used to be normal on me is getting snug, tight, and small enough on me that it ends just right above my belly button. I also had to loosen my pants. even my curves are disappearing. I am just... round now. By the way, I AM NOT PREGNANT I am just noticing recent swelling in my abdominal region and gained six pounds in less than a week. I want a diagnosis. Am I unhealthy?