I think my ex is just deliberately trying to cause trouble here...
we have a 15 year old ds together, co parented reasonably up until last year/1.5yr ago. Ex got a new girlfriend when ds was 12. I didn't mind, said I was happy for them and so long as she treated ds respectfully and with kindness all would be well and for a while she did.
However, I do know she is an ex heroin addict. Which isn't a problem so long as she stays clean. Ds came to me 18 months ago and confided in me that he believed she was using and every time he saw her she seemed like she was drunk/intoxicated to some degree. By this point his exh girlfriend had moved into his home (again, fine) but this meant that ds felt he couldn't not be around her. ds doesn't like her when she is high but had previously got on with her v well.
I sat down with exh and said look, I don't want to cause you any issue but if x is using something you need to tell me and we need to figure out the best course of action because I can't just do nothing. I cannot have my son influenced by somebody using heroin and I cannot put him in a position where he is frightened and uncomfortable.
Unfortunately he used this as an opportunity to call me controlling, a liar, cheating slut (I didn't cheat on him in fact he cheated on me but this is what he's believed ever since I got the balls to leave him.) I walked away and tried to open the topic again a few days later because it needed talking about. He flipped again. I got social services involved, contact was suspended for a month but I offered exh to come to my home or take ds on an outing with my sister and her son. He declined to do either. Social services sided with me and advised that it was inappropriate for my son to be around someone who was using, put exh girlfriend in touch with people who can help her with her addiction and since then exh has kept ds seperate from her as SS agree this is in his best interests.
I trust ds would tell me if He was around his dads girlfriend as he does not like her now and finds the concept of drugs very much terrifying.
However my co parenting relationship with exh has slowly died since then.
He keeps letting DS down last minute, eg he promised to take him to an event this weekend just gone and didn't turn up. Ds was very upset about it but as exh hadn't let me know in advance I was unfortunately headed to a work related event myself. I did however bribe my mum to take ds, so he still got to go. But that's not the point. It was exhs promise.
Now he's trying to fill ds's head with shit, to the point where ds is coming home upset telling me he wishes that his dad would stop trying to insult me and just spend time with him. It's ruining contact time and I'm told that exh will go out of his way to change the topic onto me or how unfair I am while ds is trying to chat to him about his friends/aspirations/homework/something cool he did on the PS4.
Now this week exh didn't pay maintanence on his usual day. I hadn't said anything yet but he's handed the entire amount of the next 4 weeks maintanence (not a small sum of money, by any stretch) to ds and told him not to let me have it because I'll spend it on myself. News to me, I haven't even had a basic hair cut for 2 years!
Ds came home upset and has given me the money anyway, and even said if I want to I can buy something for myself. Bless him. But not his choice, or at least shouldn't be. For now I've put the money in his savings jar, he will still receive his usual pocket money (he gets it for helping around the house and mowing the lawn for me) but I just know ex will be raring to tell ds how I've taken all of his money for myself 
Aibu to be fucking furious? It seems he's tried hurting ds by letting him down and now has moved to trying to hurt my relationship with him by making him believe I'm an evil gold digger.
I don't believe it'll work but I'm still furious