Besides stool softeners, which I still use, I also have begun to use Metamucil. I am starting to take one teaspoonful each morning in a glass of water. Again, sip slowly and take your time drinking a glass of water. You don't have to take it all at once.
As to drinking water in general, sip sip sip all through the day and not a lot at any one time.
Cramps are gas most likely. Whenever you feel the cramps starting to happen stop whatever you are doing and burp burp burp. In fact, I now stop before cramps or aches happen and burp out whatever gas is inside my stomach so I can continue the meal.
You are three weeks out so you are probably either on liquids now and/or puree depending on how you are doing and your surgeon's instructions. So, it is no surprise that you are having these minor issues. As you learn more about what you can take in at any one meal/drink you will adjust.
Initially, I couldn't do much at all either. It wasn't until I was about 5 or 6 weeks out that I finally got my water intake up over the very minimum of 48 ounces heading for 64 and more. Also, the 48 ounces up to 64 ounces is any liquid including water, shakes, drinks such as crystal light, etc.; not just water.
Ralph