The good news is that you probably don't really have a "bad" memory. What you're experiencing is a weakness in a particular type of memory (reading comprehension). However, I'll bet there are plenty of other things that you are able to remember. What are some of those things?
What if I have a really bad memory? When I read a page of a book, I can't go back and remember it.
Perhaps you're good at memorizing song lyrics or movie quotes. This might be a clue that you're an auditory learner. If the book you're reading is available in audio form, try listening to the chapter as you follow along in your book to help the meaning stick. Or, you might try reading aloud (try it at least once, even if you feel silly). Or, make up a goofy little song that covers the main points of your chapter to help you remember.
On the other hand, if you're good at remembering grocery lists or the contents of a letter that you wrote, you might be more of a tactile learner. If that's the case, then you should look for ways to include more writing or drawing in your studying. Underline the main points as you read, write an outline of each chapter, make flashcards for yourself, and be sure to take notes during class lectures. For some people it doesn't even matter much whether or not you go back over the notes later. The simple act of writing itself helps cement the concepts into memory.
Finally, you might be best at remembering something after it has been demonstrated to you. If that sounds familiar, then you should make a special effort to visualize what you're reading. It may also be helpful to discuss the chapter with someone else after you're done reading. If you're studying a novel, you could try watching a movie version of the story in addition to your reading. Diagrams can also be a great help for visual learners. Can you represent the main ideas of your chapter with a chart or drawing of some kind? You might find images easier to remember than the words themselves.
Whatever approach you take, don't give up! Once you tap into the learning approach that works best for your brain, you'll see an improvement in every class from here on out.