If someone is bullshitting you, they are lying to you. Bullshit is an enhancedway to say 'lie.' Bullshitting is a different form, turning the meaning into 'Lying.'
when you really don't want to do something or hang out with someone, so you make up excuses until they leave you alone
girl one: i thought you were supposed to be hanging out with jane today?
girl two: nah, she's bullshitting. she pretended she was sleeping when i called her even though she was on facebook right before.