How can a parent not wanna call and talk to their child?

When I called my mother on Mother's Day and her roommate said she doesn't wanna talk to us right now and she wants to me left alone. That broke my heart. I called her and she hasn't returned my calls. I was going to tell her that I'm coming to see her a couple days after my graduation next week. I wish she was here to see me graduate.
I got sad and envious seeing people with their mothers on mothers day.