I am 17 weeks along and still get it from time to time. Your body is adjusting to all the changes. The cramping is probably the placenta expanding and stretching. Since its growing it causes tension in your bowels. I am not an expert but this is what I've been told.
I'm on my second baby, and I actually had the same thing with this pregnancy. Only had the diarrhea in the evening and some cramps. It passed for me the next week. Just another part of the stuff that comes. I wouldn't stress unless you started to bleed. I'm sure it's ok though.
Any of you have experienced this? Ftm