Meloxicam does not usually cause weight gain or loss. However, fluid retention is a common adverse effect, observed in 0.6% to 4.5% of those taking it. If fluid retention is not managed, it can cause cardiovascular issues and, eventually, congestive heart failure.
One effect of fluid retention is an unintentional increase in body weight. So, if you gain weight, it may affect fluid retention. Consult your doctor right away.