i dont know whats wrong with me but i think somethings is really wrong with me.

Hi im lizzy and im 18. i weigh between 7 to 8 stone and im 5'4.

Well somtimes I end up running late for the bus to college, so I try to run part of the way but over the past few weeks, maybe 2 months, i been haveing trouble running short dissidence with out getting out breath and ended up with a stitch. Though If I jog then i can go a bit feather with out getting out of breath or ending up with a stitch. Then because I have to go up  a hill as part of the way to the bus stop.So if i haven't got my breath back or my stitch isn't gone, then I finding it tiring and hard to walk up the hill.    

I also find it hard to carry things as the other day i was carrying 3 bottles of the yop yogurt drink,1 bottle of 500 mls of water and large bar of galaxy. When carrying this I was finding it rather hevey to carry and keept switching the bag from one to the other bag. I also fell like i have very little upper body strength.

I dont know what is wrong but i fear that there is something majorly wrong as I fell that an 18 year old girl should have more strength and stammer than what i have