Heart rate question -18 weeks

I just had my 18 week monthly check up and when the doctor was checking the heart rate, he struggled to find it at first and it was inconsistent. After about 30 seconds he found it and it was beating normal. He said when I lay down, it can decrease oxygen to the baby so the heart rate can drop for a little bit. He didn’t seem concerned, but that has never happened in any of my previous appointments or my first pregnancy. Is that something to be concerned about?? I also have been cramping a lot like period cramp and he told me to drink more water.