first of all i eed to know your the difference of days between two visits and which trimester u are.i.e. no.of weeks of pregnancy
again body wt.is not a reliable indicator of the status of ur baby.
there might be a possibility that different weighing machines gives diff.results.
still if not sure u can do usg for the status of ur baby.
anyways just check or the weight properly the next time around.
if even after a month ur weight gain is less than 1 kg then it warrants caution and further tests to rule out foetal growth restriction.