Coworker friendship suddenly feels different and I don't know whether to ask about it or let it go

I started a new job at a large corporate company in December. I'm an external contractor and many new people were hired at the same time for a new project.

This was a completely new environment for me. I've never worked in such a large company or with so many people. The first few weeks were stressful.

I don't mean this in a judgemental way, but many of the people I started with were very loud, opinionated, and shared a lot about their personal lives from day one. I'm an extroverted person, but I have a full life outside work and I've never been someone who looks for close friendships at work.

A few days into training I randomly clicked with another woman who was also new. We joked that we were the only "normal" ones and ended up on the same sub-team. We naturally started sitting together on office days and got along immediately.

For months we talked about everything. We laughed a lot, vented about work, shared personal stories about our families, therapy, relationships, mental health, and everyday life. She is a very direct person who always says what she thinks, and I appreciated that honesty. Looking back, I think I probably shared more than I normally would, especially about my chronic illnesses, anxiety and health anxiety. At the time it felt mutual because she also opened up about very personal things.

We met outside work once to try a restaurant we had talked about, and in May she invited me to her birthday. I was genuinely happy to be invited. Her friends are younger than me and have a different lifestyle, but I had a good time, connected with several people and left early.

After that she went on a long holiday, and shortly afterwards I went on mine. Since we've both been back, something feels different.

I don't want to assume everything is about me because I know she's in therapy and has her own struggles. She recently mentioned that her therapist believes she has some borderline personality traits.

We still book seats together in the office, but it feels awkward now. She rarely asks me about my life anymore. I used to ask her about things she'd previously shared, but I've gradually stopped because I don't want to feel like I'm putting in all the effort.

Now I mostly keep conversations work-related unless she brings up something personal first. I don't feel comfortable opening up anymore because I'm worried I'm annoying her.

She doesn't really talk much to other colleagues either, so I don't think she's replaced me with someone else. I've made other connections at work, but honestly I mostly go there to work. I'm not looking for deep workplace friendships.

The thing is, sitting next to her now makes me sad because it feels like I've lost a friendship without understanding why. I'm also dealing with depression and other personal issues at the moment, so I don't know if I'm being overly sensitive or accurately picking up on a change.

Part of me wants to ask if everything is okay between us. Another part thinks I should simply respect the change, accept that people sometimes drift apart, and stop looking for an explanation.

Has anyone experienced something similar? Would you ask directly, or would you let the friendship naturally become more distant?