It could be a number of things, all have been stated here. If you're under a lot of stress or are a heavy, heavy drinker, you maybe getting an ulcer.

I developed ulcers when I was a drinker. The last time I got drunk I ended up in the hospital vomiting and shitting blood, after 6 beers. After a good 4 yrs straight of abusing the junk, the doc. told me, "keep drinking and be dead when you're 30, or quit, your choice", so I made the obvious choice.

I rarely get bad, bad stomach aches anymore as I have given up drinking 2 yrs ago now, but when the feeling does come, it's a mix between being burned on the inside slowly with a cigar, and someone taking a pair of needlenose pliars and twisting my stomach. It is VERY painful when it happens.

Just go to the doc. and get checked out as it could be a number of things. If it is your Appendix GO NOW. If it explodes and releases that deadly poison in your system it will be a painful death for you. The Appendix can not be ignored, so I'd go right away.