Feeling hurt after attending friend's baby shower - need advice

Hi everyone, I could really use some advice on a sensitive friendship situation. I recently attended a baby shower for my friend Sarah, who I've been close with for years. Another friend, Julia, wasn't invited and is now upset with me for going. I'm dealing with some ongoing health issues, so getting out to socialize was really nice for me. But now Julia is making negative comments about the other guests and saying they're all 'fake'. She keeps emphasizing that she doesn't care about not being invited, but her actions suggest otherwise. I feel like she's unfairly making this about her feelings when it was meant to be a celebration for Sarah. Julia often gets upset when I spend time with other friends, to the point where I avoid mentioning my plans. I'm not sure how to respond to her texts about this without damaging our friendship. Has anyone dealt with a similar situation? How can I set healthy boundaries while still being a supportive friend? Any advice would be so appreciated!