The reason for his interest flagging doesn't matter. Not one whit.
All that matters is he's not too into you. You proved to be a roll in the hay and that was, like you said, probably not the greatest for him ever. So he got what he wanted and moved on. He may not even have wanted that initially, but then once the opportunity presented itself, he figured, what the hell.
Either way -- a few things.
- Don't chase anyone. Sending an occasional note is one thing. But cut the every day stuff until you're in a more committed connection. You weren't. So expecting him to contact you every day led to unnecessary disappointment on your part. And on his, it probably felt smothering.
- Please don't sleep with people who don't seem to be that into you. It won't magically turn them attentive.
- Find lots and lots of ways to meet people. And I said people, not men (although single men are a subset of course). Get out in the world and go to classes, the gym, volunteer, whatever floats your boat. Be among people, stick your hand out, and introduce yourself. Why? Because this guy is small potatoes. When you have a number of relationship options, you'll see why this is so.
- Please get yourself reliable birth control AND carry condoms with you, every day. Just stick 'em in your purse. Now that you are sexually active, even if that's infrequent, you need to prevent both pregnancy and STDs. Talk to your doctor about birth control options as there are plenty out there.
- And, hang in there. They can't all be gems. Take a lesson from this in not chasing or bothering with someone who's lukewarm, ever again.