weight loss concern?

im 5ft 8 and have recently been loosing weight , i have gone down from 8st 13 to 8st 7 , and recently now i am 8st 5 , past year ive had tests done and nothing has come up yet im still loosing no matter what i eat.

on a normal day ill have a protein shake and a bowl of cereal with skimmed milk , 2 pitta breads with ham (quorn) and salad , for tea ill have stirfrys and quorn chicken etc , and another protein shake , then ill have supper.

i dont go without and i eat , chocolate and sweets etc.

i am an athlete and i run 3x a week sometimes four , averaging 3 , 5 mile runs and 1 , 10 mile mile run a week.

is this anything to be concerned about or is my exercise making me loose weight?