That's a pretty low dose of prednisone for only being on it for 3 weeks since all of this started. It's important to get the bleeding under control to avoid even more complications from Crohn's.
Your Crohn's may be in a state where Pentasa is not strong enough to get things under control. Sit down and talk to your doctor. Ask him questions and make sure you have a good understanding of your situation, what part of your body is affected and what the likelihood is of your condition worsening fast without treatment especially since you are pregnant. Ask about your other medication options, diets you should consider based on your symptoms and other lifestyle changes that might benefit your condition and this risks all of this has on your developing fetus and base your decision on that. If you are not happy with them, consider getting a second opinion.
And yes, stopping prednisone too quick or too early is very dangerous. When you start taking steroids, they take the place of your adrenal glands operating. Being pregnant makes stopping medications suddenly a more risky situation. Your doctor needs to help you wean off of this medication to avoid adrenal crisis which is a life-threatening condition.