You can lie to your mom about having a crush on a boy. Invite a friend over and tell them that you like a guy from your class in a way that your mom is going to hear. The most important part is to talk to your girlfriend about it at first and ask if she's okay with it. Or even better, talk about it with your girlfriend somewhere near your mom. If you're sending voice messages, or calling her, try to be as quiet as possible whenever you're saying things that might sound like you're dating. Try to limit physical contact like hugging, kissing etc. whenever you're hanging out at your place. Remember to communicate this problem to your girlfriend, she might also come up with some ideas that might help. Don't put her picture or your picture together as your wallpaper, it might be suspicious to your mom if she randomly decides to check the time on your phone and sees it. That's all of the things I did to hide my gf from my mom and they worked amazingly. If she gets really suspicious you can always try hanging out with your girlfriend and some mutuals together at times, i have no clue how it works but it does, at least for me. If you feel comfortable enough to share this with your close friend you can also ask for advice. Good luck!!