Buttock muscles? Ouch, that sounds painful. Not a great time to be going into hospital with an injury like that but it's fortunate that it wasn't a month or so ago. I always knew gardening was bad for you, along with exercising.
Seriously though, it takes a while for those to heal. Even simple strains are painful for several weeks, and there's not a lot you can do except let it rest. I have to remind patients of that every now and then.