I have been having digestive problems for a while: stomach pain, constipation, bloating. When my digestive problems are at their worst, I feel dizzy. After a large bowel movement after being constipated I will feel dizzy too. My doctor said that this is because of pain. I don't know if that is true. I am not usually in severe pain when I am having digestive problems and dizziness. There is usually a little bit of pain, but i've had much worse pain other places in my body without getting dizzy.
Sometimes digestion and bowel movements associated with constipation require a lot of blood to go to the place in the body where it's needed. This can cause a very temporary loss of blood (and oxygen) to the brain, leading to light-headedness--or even fainting. My surgeon at the Univ. of Chicago told me this after it happened to me (a few times a year after gastric surgery).
If you are dizzy 5 or 10 minutes later, I might be more concerned, but if you are dizzy within a minute of the bodily stress, that's probably it.
BTW, my favorite cure for constipation is taking 2-4 oat bran pills twice a day (available at health food stores, not pharmacies). And they are healthy for you even if you don't need them.