Well I agree with those who posted above this one but also know that it is a difficult thing to do. It might be easier for you to write your mom a note or an email explaining that you want her to be proud of you and that you know how important grades are to her. You can explain that you didn’t know how to tell her your grades because you felt ashamed and knew she’d be disappointed. Then you could also tell her what you plan to do in an effort to improve. It may seem braver to say these things in person but it is also harder. And, ultimately what you want is for her to know the truth. Also let her know that you were working hard and the steps you did take to try to do better but that you will take different steps in addition in the upcoming semester. I’m sure she will be rooting for you.
Also, seek out support at your school. use their resources. Talk to an academic advisor. You also might want to seek out help from a counselor to get you over the hump early in the semester. Don’t just sit on your angst. There is a reason schools have resources-they are there for you to use. And they can be very helpful.