I have my issues such as depression, anxiety and C-PTSD (not diagnosed yet). I want to know how I can communicate this to people online who ask me to join in with stuff.

For example, I have a group of friends that get together on some days of the week and play games together, often one or two of them will ask me if I am interested in joining in, which I usually am and it's really fun, but on occasion I don't feel like it because I am triggered that day. 

What can I tell them that doesn't sound like I can't be bothered? Because they can see that I'm online still and that I am just playing by myself... 

I am probably worrying about this far too much, but it's because they keep asking me, and I know that is usually normal in this situation, and that I should have probably joined them by now...