What Causes Low WBC And Vitamin D Levels?

A few months back I had a blood test done with results showing my white blood cell count low at 3.8 and vitamin d levels also low at 10. The doctor has ordered another blood test and if still low wants to run more tests. I m stressing about what this could possibly be from (the low wbc count). Should I be worried about cancers or HIV. I ve been sexually active with one partner who has previously been tested negative for hiv. I did get a tattoo a few years back. Is it possible I contracted hiv through the needle? Thanks.