35 weeks pregnant and im having headaches when i lie down?

I'm currently 35 weeks pregnant and for the last 4 or 5 days I've been suffering from headaches when I lay down to go to sleep. Even if I doze off when I wake up the headache is still there. It is becoming very annoying. I've been taking tylenol but they only make me sleepy. I also realized that if I sit up or stand up the headache pain goes away, only to return if I lie back down..its hard enough to sleep already. What could be going on? I want some rest.