Speaking from experience, it’s a sign that your work life balance is out of sync.
Our friends are there for us when we have stressful times but if that’s become the norm then it will be tiring for her and she’ll be feeling as stressed as you are!
It probably took a bit of courage on her part to mention how your work is affecting her so don’t stop calling her. Instead use it as an opportunity to get away from work. Meet up for a walk or cinema or something that helps you to leave work behind for a few hours. Ask her about herself, talk about what you did in your time off, what’s on TV, anything. Sounds like she’s a keeper but needs cheering up.