My girlfriend goes through periods of being very affectionate to untalkative. Advice?

My girlfriend goes through these phases where for days, I can do no wrong. She calls me perfect, I’m the best boyfriend ever, I love you so much, I feel so lucky, etc. Then, seemingly out of nowhere, she’s less responsive, less talkative, still says “I love you” and stuff, but isn’t as affectionate. They usually follow a period where we’ve spent a lot of time together. I’ll ask her if everything alright and she says “yeah.” It’s during those times that I almost feel like I’m annoying her with conversation. This phase usually lasts 2-3 days, and then boom I’m back to being “the best boyfriend ever.” I just don’t understand where these phases of disconnect come from, and it bothers me. I’d ask her, but I already have, and she insists nothing is wrong, she usually just says tired. It bothers me also because she feels like it gives her a pass to not return a phone call for hours, but if I go an hour without replying to a text she gets upset. Advice? Are any other ladies like this? Is it avoidable or just part of being in a relationship?