Trembly duck

[Ericzahel]

Hi guys I have a 3 year old khaki female who will.randomly get body trembles ,like a shiver that starts from her tail feather and up to her head ,she'll do this a few times a month ,sometimes when she does it she'll do it before or shortly after laying her egg ,im not sure what it could be ,maybe nothing to worry about but I get worried she's my baby, she is on kalambach duck n goose feed and always has a dish of pulverized oyster shells available 24/7 next to her food ,I also give vitametz about 3 times a month as per the instructions on the package say to do 3 to 4 times a month or in times of stress or heat ,I've tried researching about what's going on and found nothing ,could it just be normal, maybe a vitamin deficiency, but if it was a vitamin deficiency I'd imagine it would be happening more then just a few times a month ,I only have 2 ducks her and her sister they are both 3 hatched only 4 hours apart and she doesn't do it , they pretty much lay everyday maybe skipping a day or 2 here n there ,they both just started laying again in February after stopping mid November due to a huge molt they both started at the exact same time !! Any suggestions will be greatly appreciated, thanks in advance