Basic Relationship Problem?

I've been seeing this girl for a month now, and Saturday was our fourth date. The first three dates were amazing; we never ran out of things to talk about, they lasted 7 hours each, and it moved pretty quickly physically. We've texted every day since the day we met.

The past week, she was bringing up future sex, and the fact that we're seeing each other. Which I thought was great. And she kept bringing up our fourth date.

Saturday was our fourth date; we were both tired from the night before (I had work, she was up til 8 AM with a friend who was leaving) and it was really crappy. The chemistry was gone, we were searching for convos, it was not fun. So she leaves at 8:30 cuz she's 'tired', and I don't hear from her.

Sunday she didn't text me at all, and today she blocked me on Snapchat.

I texted her this morning a little inside joke, and I sent her a Snap and she then full on deleted me from Snapchat.

So I give her a call a few hours later to see what's up. Of course, she doesn't pick up.

After I admit defeat, I text her that I was sorry for not being my usual self on Saturday and that it's alright if she doesn't wanna see me anymore. All i asked was for the reason why.

She responds an hour later saying she wasn't into me. Fact, she'd hadn't been into me for a while now, and wanted to end it.

I don't get it. What the hell did I do wrong. I have no idea where this came from. Advice?