Darrell Sheets' Girlfriend Told Cops They 'Fooled Around' Before He Died by Suicide

It sounds like Darrell Sheets had sex one last time before he died by suicide ... at least according to his girlfriend.

According to an incident report obtained by TMZ ... a woman who identified herself as Darrell's girlfriend told police they "fooled around" right before his death.

She also told cops the two made chicken fajitas and weren't arguing on the night he died.

The woman told police she went to pick up Darrell's granddaughter from work ... and when she got back, she texted Darrell and asked if he wanted her to sleep in the motorhome located in the garage because he was trying to go to sleep.

She says Darrell told her he wanted her to sleep in bed with him ... and she told cops, when she got into bed, "Darrell wanted to 'fool around' so they 'fooled around.'"

She says she fell asleep, and he wasn't in bed when she woke up to go to the bathroom ... so she went looking for him. She says she saw him standing in the doorway of the office, and he tried to tell her to go back to bed. A short time later, she says she heard a gunshot.

Darrell had been having a hard time sleeping, his girlfriend told cops ... and she says there was strife between Darrell and one of his sons because the son hated her. She told investigators the son left days before Darrell died ... which "devastated" Darrell and made him "sad as hell."

As you know ... Darrell died back in April amid claims he'd been cyberbullied. 911 audio captured his girlfriend frantically telling cops she thought the "Storage Wars" star had killed himself, and the death was ultimately ruled a suicide.