Is it possible for a girl to gain 13 pounds in 3 months?

my girlfriend went to the doctors the other day and he said she gained 13 pounds in 3 months. she goes to a trainer to work out atleast 2 days a week and she works 3 days outta the week and doesnt really follow her diet the way she is suppose to because she does hair and she has a curvy body type any suggestions as to why she gained instead of lose?