What if your mom is interrupting because she secretly wants to be part of your workout? She might see you doing something new and feel left out of your day. Try inviting her to stretch with you for just a few minutes before you start. Then clearly say you need the rest of the time alone to keep your energy up. This gives her a way to connect without ruining your focus. You set a kind boundary by including her first, so it feels less like rejection.
Wait, I read somewhere that giving in to interruptions can accidentally reward them. If you include her first, she might start expecting it every time. Setting a clear start and end time for togetherness works better.