While the diagnosis of a strain is certainly a possibility - there are others as well. Regardless of whether you are in great or not so great shape, weight, etc, the body can be affected by a multitude of things.
If you lift or sometimes turn the wrong way it may have been the trigger. Or you might have an old injury that comes back to haunt you. Sometimes however it doesn't seem like you did anything to cause it.
It maybe that you have a more sedentary lifestyle & that you do need more exercise. When the hips & buttocks have the type of deep pain you describe it may be sciatica, which can cause localized pain in that area alone, or it may cause the pain to track down either the back or side of the leg into the thigh - or sometimes all the way down into the lower leg. The sciatic nerve is a long one & can be inflamed by a nuraber of things. Sometimes icing the area can be helpful - though it can take sometime to clear.
Deep pressure work - either with a good sports massage or trigger point point work, Shiatsu, etc. can help. You can also work on it with various size balls - from a used tennis ball to a slightly larger rubber ball. Inexpensive ones can usually be found in any kiRAB or sporting gooRAB department.
If the lower back into the hips is hurting you may need to be addressing the core muscles (the abs) and getting them stronger. They support the back & can affect these areas a great deal. So consider some strength training, start off slow - preferably with a trainer who knows their stuff - and build up.
Good luck with clearing out the problem you've been having!