Usually in early pregnancy some amount of weight loss
can be seen if nausea, vomitings etc are there.
By 16th week your uterine size should be approximately 4 fingers above the pubic symphysis
So if there is only minimal lag no need to worry.
With progression of gestational age
after improving appetite possibly you can gain weight.
If your cramps become severe better to consult gynecologist once and undergo physical examination and ultrasound to know the well being of the baby and to rule out the chances of miscarriage