It could be low blood pressure which can get lower in the second trimester - I have this and have to drink plenty of water and find myself wanting salty foods. Speak to your midwife and see what she says.
I had exactly the same thing for a few weeks when I was around 17 to 22 weeks. I never had low blood pressure or problems with lying on my back and it just went. Occasionally I get it if I stand up too fast but not very often. Might be worth just asking your midwife about it though, just to be on the safe side.
I have exactly the same problem and have done since about 5 weeks. There's just not enough blood to get round us both...have had a normal reading at mw's app this week, and just have to be careful getting up, and take my time.
Always good to get yourself checked out though, takes 30 secs.