S. -
Hello and Happy New Year! My son, who is also 15 months old, went through this about two months ago. He used to love baths, then my husband's work schedule changed which threw off everything else in the house, and it became easier for us to shower with The Kid. Well, after things calmed down about three weeks later, I tried to give him a bath one night and he COMPLETELY freaked out! Didn't want anything to do with the tub! It was puzzling because as I said, he loved taking a bath. So over the next several days I hyped up the bath. I let him stand in the bathroom with me while I drew his bath instead of drawing it then bringing him to it; I undressed him in the bathroom; I made the prospect of getting in the tub more enticing by putting his favorite toys in it (he's particularly fond of this yellow ducky); and finally, I started by only running a little bit of water. Each day I would run a little more, until finally a proper bath was drawn. It took about a week or so for him to warm up to bathing again. An added bonus - we used to have such a hard time preventing him from drinking the bathwater. Since the change in behavior and consequent wooing back to the bath, he hasn't shown any interest in drinking the water. He tried to one time I think, but I told him no sternly and he hasn't bothered to try since.
Hope this helps! This (behavior) too will pass!