If someone is bullshitting you, they are lying to you. Bullshit is an enhanced way to say 'lie.' Bullshitting is a different form, turning the meaning into 'Lying.'
"Larry, don't bullshit me! I know you were at a party last night!"
Larry is bullshitting with the excuse of going to the library.
when you really don't want to do something or hang out with someone, so you make up excuses until they leave you alone
girlone: 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.