sudden mood change in 2year old?

[mizznewman90]

My son has just started going to "time for twos" as he doesnt really mix with other children (i have no friends with kids or any family close with kids either) before he started going he was happy and always giggling, if he was told no then it wasnt a problem i just got "yeah".
Since hes been going when i tell him no, he just pretends to cry or he has even smacked me.
So far my current way of dealing this is to kneel down so im at his height, hold his hands, ask him to stop crying and tell mummy whats wrong. this is no longer working and i was wondering if anyone else has any suggestions on how to stop or just curb this behaviour xx