I could be that you just had a bout of low blood pressure, sometimes happens when you lie down for a while then get up too fast. 38 bmp is a bit low but if the doctor has not been worried by it you should not be either.
If you have no other symptoms don't worry. Just so you know every one has a different pulse rate. If you are fit and train regular then a low bmp is fine, mine fluctuates between 40 and 60 bpm at rest.