Past tense of "bulls**t"?

bullshitted.

bullshitted is the past tense and past participle of the word bullshit. although you can use bullshat as a past tense but bullshitted just rolls out the tongue better (especially when some bullshitting is involved).

As for the past tense of shit, there is shat or shitted, but the commonly used form is shat. It is, according to Grammarist,

the widely accepted past-tense inflection of shit. It was formed by analogy with the verb sit, which becomes sat in the past tense. It was originally a humorous and slightly sanitized version of the curse word, but it has become the standard form.