My back is killing me!

I am partially done with my errands but my back is killing me already! Is this the sign of getting old? or maybe because I don't have enough calcium in my body, whatever it is, I need to know because it is not fun when you are in pain, I never did heavy things today, I only vacuumed the floor and then laundry. Hubby called me today that he will be home late which is also a good thing so I will have enough time to make his dinner and relax for a little bit, sigh!