first and foremost, I am not a doctor or a physical therapist.
However, based on what you said it sounds like your lower back is being overworked, ie, it's working extra hard to support your frame. I don't know if this is caused by slouching or something else, but doing some exercises that strengthen your lower back could be a good idea if you are someone who doesn't get a lot of exercise (and lapping the park doesn't count).
Get familiar with an exercise known as the Plank (or Bridge). It's a great lower back exercise. Try to get into a structured exercise program two or three times a week that includes a WARM UP (very important), several exercises, and a COOL DOWN (again important). Because I have no certification/accreditation I'm not going to get into specifics, but I recommend you look into planks as a way to build lower back strength.