I don't know, but I think this is a common anxiety that a lot of folks have. I've had the same therapist for just over 5 years now and I absolutely and 100% trust him, feel at ease and comfortable talking to him about everything and anything, and I still freak out a little when I have to call/email/text him to set up an appointment or fill him in on something. I mean, seriously? After 5 years? And he's never given me any reason to freak out or feel anxiety? You'd think I'd not feel like that. I will say that it's gotten a little better now that he's in private practice and I don't have to call and jump through 90,000 hoops just to get a message to him, but I still get a little freaked out. Sometimes, like yesterday, I get a lot freaked out. For me, it's at the hardest when I'm in crisis and active suicidal ideation. It takes a LOT for me to call or text him and request an appointment. I wind up stewing about it most of the day and then finally get so miserable that I can't stand it and wind up either calling him or texting him OR I wind up half a heartbeat from attempting.
In short, there's nothing wrong with you. You've just got some anxiety about it. This might be something to discuss with your therapist when you see him next. If he's a good therapist and has a good track record of making you feel safe and okay to be vulnerable with him, then I can guarantee he's going to do his best to put your mind at ease. I've talked to my therapist about this a few times and it's helped lessen the anxiety. It's worth a shot. (((hugs)))
~Ana