Because of your age, it sounds very much like IBS, which can give you the symptoms you describe. I was diagnosed with this at age 29, and except for the watery yellow stools, I had every symptom you mention. The cramping could very well be colon spasms, which are quite painful. I had two barium enemas back then, about a year apart because my symptoms didn't go away and I went to another doctor, but both tests came back clear so based on that and my symptoms, it was diagnosed as IBS, or spastic colon as it was referred to back then.
A barium enema is not a big deal, and most of it depends on who is administering it - my second one was alot more comfortable and alot quicker than the first. The day before you will be on a clear diet and will also have to use something to clean yourself out. You do not want anything left inside that will show up on the x-ray as 'something' so being completely cleaned out is important. The test itself is pretty simple - they will insert the barium into your colon, then you will be asked to lie in different positions and the table will turn this way and that....this is to get all the different x-rays they need. Again, not a big deal. After the test is complete, you will be allowed to go to the restroom to get rid of the barium and then go home. The most 'discomfort' you will feel is hunger from not eating the day before