I ditto K&K. I would start with a vet check first.
I recently had to get some soap for Stormy who is unwell. I had never used soap on my buns but I read this article: http://www.bio.miami.edu/hare/buttbath.html
Use a hypoallergenic, non-medicated shampoo. Most popular shampoos for humans–including baby shampoos–are not recommended for use on rabbits, as they can contain harsh ingredients that are rough on delicate rabbit skin. Pet shampoos containing pyrethrins and other “herbal” or “natural” insect-killing ingredients are not recommended, either. Plants make toxic compounds to kill their herbivore pests, and just because something is “herbal” or “all natural” doesn’t mean it’s safe for a bunny! The best shampoo (for you or your bunny) is an organic, environmentally-friendly (and cruelty-free) product with emollients to soothe the skin, with as few additives as possible. (Bunny-safe shampoos are easy to find at natural food stores or other vendors that sell environmentally friendly products. Read the label and be a smart consumer.)
I bought a bar of one of my favorite human brands “Nature Clean” (chemical and cruelty free!). I just swish the soap in the sink so there’s just a bit of suds and then rinse really well.