I just looked up my levels. Mine tend to run on the low side of "normal".  

I looked up low sodium levels.  Here is a link to the Mayo Clinic.  Have you read up on it?  It appears some medications can cause low blood sodium levels.

The other question that I have:  have you been testing for blood sodium levels in the past?  Do you historically have low sodium levels?  Personally, I don't think I would make a change based on one data point.....i.e. if you have been having blood sodium in the range of, say, 135-138, and then you have one test below 135, I would monitor the situation, but not make a change until I saw a definite pattern in the blood test levels.. It is entirely possible this could be an outlier, especially if your levels have been in the normal range in the past. That is how I would see it.