I have been on humira for a couple years and I have had some weird side effects as well.
The burning or pain of the shot has never lessoned for me. But I tolerate it because I do feel better.
I don't get stomach cramps, but get horrible gas for a day or two. Nothing seems to help, but I just deal with it (and so does my hubbie lol).
I hope someone else can tell you if they have cramps or not.
Humira has been a life saver for me so far.