I am not a plumber nor do I play one on TV! I would suggest a plumber first. It could be a simple leak within the wall pipe coming into the bathroom. If you have a cinder block exterior or brick wall below grade, water could be seeping in. Check the wall from the outside, dig down below grade to check. A good dryloc product or hydrolic cement may do fine. If you have a framed exterior wall, I would check to see if your house was constructed with a perimeter french drain system. If not, french drains are fairly cheap to install yourself, just lots of labor hours. So, first choice, call a good plumber, second choice, call a good contractor. Brian Kulis, http://kulishomemakeovers.com
Brian Kulis
https://kulishomemakeovers.com/