Losing weight.

When I went into my doctors appointment on March 3rd I weighed 228, I then went in a month later on April 4th and weighed 221. The other day I weighed myself and I was 219 and today I weigh 216. So overall within 5 weeks I've lost 12 pounds and I'm a little concerned. I'm pregnant with twins and I'm 18 weeks and 2 days. Just curious if anyone else had/has weight loss while pregnant. I wouldn't be so concerned if my doctor wasn't concerned about it at my appointment.