Wow, I get that exact same thing. However, mine started before i fell pregnant. I notice it more in the mornings too, only when laying. If i sleep on my left side, i feel it in my left lung, if i sleep on my right side, i will get it in the right lung. But it goes after i get out of bed, after coughing, but sometimes coughing doesnt make it move. When i first started to get it, it actually felt as though water was running up my lung when i took a deep breath. It got a bit better when i was preg, but now that i have had my baby, it has come back again. I too wish i knew what it was.
I had chest xray, and a test for asthma, and both came up normal.
What does your Doc say? Let me know how it all goes.
Take Care