This may happen because she is concentrating so hard on the decoding (sounding out), that she is unable to remember and comprehend the full sentence. This is a good indication that the books she is reading are too challenging for her at this time.
The next time you and your child choose books, you may want to ask her teacher, a librarian, or a reading specialist to help you find "just right" books for your child. These should be books that your child is interested in and that she can read with about 95% accuracy the first time. Ask her to read a page or two aloud while you silently count the errors from the total numbers of words on the pages she reads. This will give you an estimate of her accuracy.
By reading "just right" books, your daughter will practice all aspects of reading, including fluency and comprehension. And comprehension, ultimately, is the goal of reading! As she reads "just right" books, her ability to decode words will become even more automatic. As she gains proficiency, the text will become more meaningful because she will be able to understand and enjoy what she is reading.
Check out Reading Rockets' recommended books by theme for some ideas!