Too soon?

So last night I went out with a guy who I've been seeing, it was the third time we met and we ended up having sex, I'm 16 and he's 18 and I do regret it, originally I did want to wait until a relationship started between us but at the time it was sort of a oh I'm in the mood so I don't care thought. I've noticed him sending less & less messages since lastnight or I could really just be overthinking it but I'm stressing out, we've been talking for over 2 months now but to be honest I don't think I even really enjoyed it, he did go for a long time so does that mean he didn't enjoy it that much either?