What do I say when my partner says "I love you" and I'm not ready to say it back?

My girlfriend said "I love you" while we were walking home. I squeezed her hand and said, "I'm really happy with you." She smiled, but the rest of the walk felt quieter.

We've been together three months. I look forward to seeing her, and I want to keep building this. The words just feel bigger than what I can honestly say yet. The next morning she told me she hadn't expected a matching answer, though she hoped I wasn't avoiding one because I already knew this wouldn't last.

I don't know that. I also can't promise that waiting another month will produce the answer she wants. Should I bring it up again and explain more, or would that turn a loving moment into a discussion she didn't ask for? I want her to understand my pace without asking her to wait indefinitely for a feeling I can't schedule.