My GP told me that roughly half of her patients, of all ages, have Vit D deficiency because we don't get enough sunlight on us. Many people also don't drink nearly enough, and are dehydrated, which can cause tiredness. I would definitely recommend having a word with your doctor about further tests. It sounds to me, though, that your main problem is too many responsibilities. It's difficult, when you are a carer, to step back and take care of yourself. It's not just physically tiring, but mentally, too. Try to make more time for yourself, if you can. Tell your family that you're getting older, and can't do as much as you used to.
I'm 83, and don't care for anyone but myself. I sometimes get tired, but try to go for a walk every day. It's easy to become a couch potato, if I'm not careful. My main hobby is singing. I'm only in two Community choirs now. Not too demanding, but it's amazing how a good sing lifts your spirits and sets you buzzing. The social side is good for me, too. Why not give it a try? You don't have to have a good voice