Unsure of the "Rules"

I'm sitting here at work with a million things to do, and only one thing on my mind. I haven't heard from A in over 5 days. I know for many that wouldn't be a concern, but I used to get at least a text message every day. I've sent 2 messages that he didn't respond to but chalked it up to back to school busyness however, now I'm starting to worry that I've done something to upset him. I don't want to send the stupid "did I do something wrong?" email or phone call for that matter, and I hate obsessing over it. I check my messages every 15 minutes I'm sure, and each time his name isn't there, my heart sinks again. The scariest thing for me is that ever since he left with his mom after their visit(in August) his messages (the ones he has left) have been very short. I hate feeling this insecure about things. I hate that I worry all the time that I'm going to lose him again. My heart physically hurts - that can't be good for a person. I think the worst thing is that I don't know what the "right thing" to do is. Do I send him quick little emails just letting him know I'm thinking about him (which is what I'm doing) or do I give him space and wait till' he contacts me (then I feel like I'm playing games)? I just so desperately want to make sure that I'm not upsetting him or making things too difficult. I need a reunion rule book - something to guide me through.