Tree trunk calves...

[Dianyla]

Have you been getting enough potassium lately? If you've been training a lot and sweating you've probably been losing a lot of electrolytes. Sodium is usually replaced very easily because it is added to so many foods, but potassium is not as ubiquitous. Bananas are a good source, as are Larabars (primarily due to the dried dates ingredient). I also like Morton's "lite salt" which is half sodium chloride and half potassium chloride, you can find it at any grocery store.

Magnesium is another necessary mineral required for muscle relaxation. This is why soaking in a bath with epsom salts (i.e. magnesium sulfate) is so relaxing. It's also why too much magnesium has a laxative effect - in high doses it can relax the sphincters in your gastrointestinal tract. Chocolate is high in magnesium.

So, go eat a few chocolate dipped bananas and you'll be fine.

ETA: Just noticed your edited in question about how do you know if you're lacking potassium? Well, cramps, for one! Another way to look at it is that if you're not consuming potassium rich foods or sports drinks and you are sweating a lot, you are most likely deficient in it. And if you lose a lot of electrolytes due to IBS, this is also a factor.

ETA2: Another possible source of cramps is taking calcium supplements that don't also have magnesium. Cal-Mag work together in muscular cell contractions, the calcium ion helps with contractions and the magnesium ion does the relaxing. Too much calcium unopposed by magnesium can also cause cramps.