I do something similar and after 7 years I discovered I was severely Vitimin D deficient. Thanks to an article that was posted recently about UV not penetrating glass.
Do you work outside in the sun? Do you eat a lot of fish? Do you take a vitamin D supplement? If you answered "no" to all of these questions then you probably have a vitamin D deficiency. Vitamin D deficiency is very common.
>The National Health and Nutrition Examination Survey 2005 to 2006 data were analyzed for vitamin D levels in adult participants (N = 4495). Vitamin D deficiency was defined as a serum 25-hydroxyvitamin D concentrations ≤20 ng/mL (50 nmol/L). The overall prevalence rate of vitamin D deficiency was 41.6%, with the highest rate seen in blacks (82.1%), followed by Hispanics (69.2%).
I've found for me personally that vitamin D supplementation helps, but my tendons/muscle attachments are still weak and injury prone unless I get some real sun.
Getting less than 5 minutes of sun per month and never eating any foods that contains Vitimin D such as fish for several years might be to blame.
This isn't something you should self-medicate. It's tremendously simple for a doctor to confirm your hypothesis.