I've been having this problem too. Scares me when I have a headache for more then a day because those usually lead to migraines. Knock on wood, I haven't had a migraine yet, but just the thought scares the crap out of me. I've been taking Tylenol and drinking a cup of coffee and it helps sometimes and other times not so much, like today. I had OH pick me up a cup of coffee on his way home from work so I'm hoping that does the trick. As far as caffeine consumption in pregnancy, it's fine in moderation. My doc gave me a limit of 150 mg's a day.
I hope your headache subsides. I guess if it makes you feel any better (I know it helps me put up with the headaches a little better) just remember it's a side effect of pregnancy and any side effects/symptoms just means everything is ok. Hang in there. (((HUGS)))