My two coworkers and I used to go for coffees and walks together, but about three months ago, I spotted them having a coffee without me and when I asked them why they left me out, they denied it had happened before and they were discussing something private and not to worry. But ever since then ive noticed they don't want to hang out all three of us.
I've asked multiple times what the issue is (individually,I'm not calling a meeting about it or anything!) but when I challenge them they claim they're simply busy? This seems unlikely to me as the timing is off. Why suddenly become busy after I caught then out?
They sit a bit away from me and I've noticed they sometimes speak in low voices. I've become sure they're talking about me. But I've asked them and they say it's not true and that they simply are talking about something private 
Why would they suddenly start leaving me out? It's affecting my mental health and it's extremely upsetting that they keep denying it.
Aibu to think they can't know how awkward they are being? To have friends who suddenly don't want to chat as much/ are mysteriously busy as soon as I pull them up on anything? It just seems cruel and blatantly nasty?