If you want my advice, then I think that you're on the right track: Burnout. It happens to all of us, even the best of writers. Sometimes you just feel like you wanna write, sit down to do so, and instead of feeling like the normally fun and exciting hobby that it usually is, it instead ends up feeling like a chore. I think that's the difference between burnout and writer's block, in my eyes at least, anyone else can feel free to disagree.
Burnout is what happens when you feel you have an obligation to write/join in on the fun if say it's a group story/RP. But when you start to get right down to it, it feels less like you're writing a post up that's supposed to be inspiring fun and instead feels more like 'work' which RP should never feel like. If it does then you need to take a step back and re-evaluate what the RP means to you and if it's worth forcing yourself to trudge out a post when you're not having fun.
Writer's block is when you really, REALLY want to write but for some reason or another, an excuse or something else comes up and stops you from doing so. One of the most common ways I've heard of getting past writer's block is rather simple: simply keep on writing and eventually you'll find your flow and gel with it. Truth be told when I get stuck, I usually just step away from the computer, turn on some synth music or Daft Punk and try to relax while thinking up fresh ideas in my head and maybe that's what you need. Something to inspire you and get you back into the swings of things, feeling fresh and rejuvenated and ready to write out some awesome posts!
I like having music on in the BG while I post as I feel it helps my brain envision the scene I'm writing out better. Maybe you could try that if you haven't been already? Hope this helps regardless. ^^
