Ask your doctor what could cause this and if it is something that you can be prevented in the future by a change in your environment or eating habits. If the rest of your panel came back within normal it could be as simple as having pneamonia or you may be developing something controllable like ashma. Your doctor will know best and be able to tell you more obviously, but only with all aspects of your panel and history. Normal questions like, what can I do to correct this, how long will this take to get back within normal, what changes should you should make, and how concerned he/she is may be good questions. Unforunetly I always forget what I specifically want to ask so I suggest that if you are nervous or worried that you write down your questions so you don't miss asking them. My Best!!
How did the rest of your blood work look? I would not get excited it is lower than the normal range but not that low better lower than higher than normal.
I am sure your doctor will tell you the same thing.