Well, nothing is impossible, they could just crop your images to remove a watermark. Make sure you save it as a jpg and not a png/xcf/psd/psb, as those files save editing history in the EXIF data. i.e. extremely easy to undo any changes.
Hi OP, in addition to adding the watermark register with pixsy.com and load your photos. You get free level membership to check 500 pic matches across www and also help to legally sort unauthorised use.