I thought he was interested in me, but I'm thinking now he isn't?

Help me out people :-( so there was a guy in the gym that I like.. Finally started speaking to him, it was mainly hi and bye in the gym but I added him on Facebook and he accepted but I didn't get a message from him.. So about a week later, I sent him a message. He replied and we have just been talking generally about our daily lives. However, he has not replied to me from a message that I sent him yesterday afternoon and he has been online plenty of times (he has also 'seen' the message). friends have been told me to be flirtatious with him.. But I don't want to put it out there too much otherwise he will think I do it too often. He obviously knows I AM interested- I was the one to add him, and I was the one to message him. He's a nice guy, talkative too and very serious about his studies. I did take a few hours in replying to him a couple of times even though I had been online so I don't know if he's seen that and playing that 'game'

. I'm gonna b seeing him in the gym tomorrow now, and I'm afraid it's going to be really awkward.
I thought he was interested in me, but I'm thinking now he isn't?
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