Wet tail is caused by a bacteria naturally found on syrian hamsters that can take over when the immune system is suppressed, often by stress. It can kill within days—if you haven’t scheduled an appointment I would do so asap.
Also keep in mind it could just be diarrhea, especially if your hamster isn’t a syrian. This can be caused by a multitude of things, such as feeding too much wet food without weaning or infection. Again, another case to see a vet so you can get an antibiotic/probiotic.
Please keep us up to do date or post on a hamster-dedicated forum— Hamster Central is mainly for those in Europe and Hamster Hideout for the rest of the world but at least on HH we get plenty of members from Europe, too.