isolating myself

For 2 weeks now I've been really depressed, like more than I have before, but I've never experienced the need to be isolated like I have been recently. Being around other people is overwhelming, I just want to be alone. I NEED to be alone. I'm trying to fight my depression by doing at least 1 small thing a day so that I'm not just in my bed. But it's so exhausting and overwhelming when I also need to think about or be around other people. I don't go out of my bedroom if my family is in the kitchen or living room, it's just too many people. I always have my phone on me but I can't open any personal messages. My friend has been trying to reach out and is asking why I'm ignoring her but I just cannot answer. It's my best friend who I've always been able to go to to vent and tell how I'm feeling but this time I just can't. I want to explain to her but I just can't so she's taking this personally which I understand. I'm just so physically and mentally tired. I just don't have the energy to care, but I don't want to ruin the friendship and don't know how to approach this. Even in person I have a hard time speaking because my mouth is heavy and doing my homework is hard because it's hard to type with my hands, my fingers feel so heavy. My whole face is heavy and hard to move my face muscles to show expression. I was at a gym class this morning, a class I usually love but it was such a strain on my mind I had to stop multiple times out of frustration to stop myself from having an anxiety attack. How do I explain to my friend why I can't talk when I don't even understand myself? I know it's not healthy to isolate oneself especially when depressed but I'm just going to feel worse if I don't so I don't know what to do. I really feel like I need to be alone to get out of this but don't want to push anyone away by doing so.