We're in the same team for the same length of time but doing different jobs.
We started off well and were friendly, there was a small group of us who arranged to work together in the office and head to the pub afterwards then he slowly started cutting me out. Stopped being as friendly and became standoffish.
A big turning point for me was when we all arranged to go into the office, I sent him and another colleague in our team the desk number I'd booked and he came in, said good morning, and sat in a completely different area from me with our other team mate (male, friends with him). We rarely go into the office and generally the whole point is to sit with your team. Plus we'd arranged a night out that night which a few of us went on and it was fine. It felt like I was publicly shunned and I was mortified. I called him out but he shrugged it off and said he didn't realise we had to sit on each other's shoulders when we came onto the office. It was embarrassing. Our other colleague had the grace to look embarrassed and tried to get me to move to sit with them which I obviously turned down. It was so petty and childish, like he was trying to assert his dominance.
There's been little things of undermining me in meetings but only when our line manager isn't there. Then yesterday he did it twice by effectively telling me how to do my job which is completely different from his and I was in the meeting because another team mate asked me for my expertise and wanted me to join. So I did to support my colleague and then I felt my professionalism and ability was called into question when he took it upon himself to undermine the points I was making and questioning my proposed next steps.
This has really affected me. Just on Monday my manager was discussing a promotion for me (other colleague is a higher grade than me, not even the same level). I've had a great week feeling like I've smashed it and then yesterday had the rug pulled from under me and I can't stop thinking about it.
I scheduled a meeting with him alone this morning about something I need his input on but I don't think I can get through it without bursting into tears!
I'm giving this far too much headspace and ruminating over it. I need a kick up the bum!