What can cause low WBC on a CBC blood test?

[Ash1057]

I went to the doctors last week because I was having upper stomach pain/burning and heart palpitations with chest pain. My doc said it was all from my anxiety but that I might also have an stomach ulcer from stress. I got a CBC done and everything came back within the normal range besides a slightly low WBC but my doctor didn't seem worried about it. This was the results with the lab ranges.
WBC 3.9 K/ul                             4.0-10.7      (L)
Neutrophils 2.20 K/ul                 2.0-7.3
Lymphocytes 1.20 K/ul              1.0-3.4
Monocytes 0.40 K/ul                  0.0-0.8
Eosinophils 0.0 K/ul                   0.0-0.5
Basophils 0.0 K/ul                      0.0-0.2

I know my WBC is barely out of range but for the past couple of years at my routine physicals it's been much higher around 7.0-9.0 K/ul. I'm concerned because I just haven't felt myself for the past couple months (chest pain, palpitations, burning stomach, low energy, loss of appetite, weight loss, shortness of breath) and I'm scared it's more then just my anxiety. I'm 21 years old and the chest pain has been going on for years but the burning stomach is a new symptom. Thanks for any info. I really appreciate it.