I am a coffee junkie, but when I was pregnant I could not drink it. Every time I even tried to take a sip it triggered something in my brain that made me think it was poisonous- strange.
Anyway, all beans have the oils in them, but the roasting process really brings them out. Darker roasts, such as espresso or french will have a stronger flavor because of the oils on the bean. Lighter roasts, such as colombian will taste more acidic instead of bitter like the dark roasts.
There are two varieties of beans used to make coffee- arabic (starbucks, millstone, equalexchange, fairtrade, etc) and robusta (foldgers, maxwell house, etc). The arabic is used for flavor and naturally has a lower caffine. The robusta, which is cheaper and less flavorful, tends to have a lot more caffine. Also, in regards to caffine, a darker roast has less than a lighter roast because the roasting process cooks out some of the caffine. Almost all robustas are lighter roasts. There is a chemical "x" in coffee (I can't remember the name of it), and it may be more addictive than caffine. It is found in both decaf and caf, and it can irrate, too.
My recommendation to all coffee drinkers is this: drink at least twice as much water as coffee because it is a dieretic and can dehydrate you. My dh used to get headaches from replacing his 8 glasses of water a day water with coffee. Symptoms of dehydration can include headaches and stomach aches. Also, as has been mentioned before, don't drink on an empty stomach because it is acidic.
Maybe, though, Whisper, you should give coffee a break for a while. Drink tea or something that does not bother you. Good luck!