Frequent motions and gas. Diagnosis.

I had low in-ano fistula and had undergone a fistulectomy a couple of years ago. The fistula has been successfully removed. However, I have been experiencing problems with my digestive system ever since. I have to make frequent visits to the loo, especially after taking meals. Also, I mostly have semi-solid stools with undigested food particles in them. The stools are sometimes dark black in color. I am also facing gas problems increasingly these days with a bloated feeling. In addition I tend to have a dry mouth especially after waking up in the morning. I am low on energy and get easily tired. My body weight has been constant around 73-75 kg. Kindly suggest remedies/any specific tests I should undergo for proper diagnosis.