I should add that if you agree you may be constipated, then every time you eat, the food has to go somewhere, so it pushes a little more on what's inside, and yet it won't come out because it's stuck. If this is true, then you could either try some enemas to get some of the waste out, or get some Baby Lax off-the-shelf in the pharmacy, squirt two of the ointment things in there, and it'll come out from that end in a big hurry. That's what I do when I get clogged up.
Small dark stool usually indicates you have some constipation, so drinking extra water, exercising and resting well, and getting enough fiber should help you along with this. Also, you could drink some Acidophilus milk for a week, it helps break down waste. You could also have a tummy flu of some kind, so if it continues beyond a week after trying what I've mentioned, you should go to the doc, in case you have an infection in your digestion, so he can treat you, because you don't want to get too clogged up and dehydrated.