That depends on your present weight and how you lost the weight, if you lost it by not eating properly that is not healthy.

(Original body weight)-(current body weight)= weight lost

(weight lost)/(original body weight) * 100 = % of dehydration (degree of dehydration)

most lost on a diet

It is possible to lose weight but gain inches because fat weighs less than muscle. If one's body loses muscle mass and gains fat, this could happen. Also, apparent weight loss may be greatly skewed by water weight, as well as the weight of any undigested meals still in the system.

If you are referring to the past tense, then yes you would say, "I have lost some weight."