headache hell... again

I'm 34 weeks pregnant and for the last few days have been experiencing headaches when lying down. I suffered from headaches early on in pregnancy and for the last 5 months they've been really good. But they seem to have crept up on me again. It seems to occur mostly when I have been lying down, so now I am worried it's a brain tumour or something. Has anyone experienced the same thing, I'm worried about this on top of everything else!