It does sound like you might have it. Basically, FMS is chronic pain and chronic fatigue. Pain everywhere. Depression and anxiety are common as well. To someone who has no idea what FMS is, I mention it's like arthritis, only pain in the muscles, not the joints. And you're always in pain and always tired.
There are a few other things that are linked with fibromyalgia that you can get, such as IBS, which would explain the stomach cramps with diarrhea. And arthritis. And well... everything it seems like. XD (To the first person who commented about the stomach problems not being a part of FMS, it can be. Not always with everyone, but it can happen. I know people who have both FMS and IBS.)
FMS is not a vitamin deficiency and whoever says that doesn't know what they're talking about. More vitamins doesn't help you feel any better or worse with FMS. FMS affects the muscles. Not your lack of vitamins.
I'm including a few links that might help you understand what FMS is. What symptoms are, what else can happen with it, etc. Basically, it sucks, you have a ton of pain-even more with the changes of weather, you get depressed, you lose sleep, you can get other things as well. All in all, it's not a fun disease.
Even more since there's no real cause or cure for it. Some doctors don't even believe that it's real.