montagPI10.jpg Heidi Montag has enlisted the help of a Los Angeles Sheriff's Department private investigator to find out whether estranged husband Spencer Pratt has been spending loads of cash behind her back, according to a source close to Heidi.

X17online exclusively has learned that the reality star hired the PI last Monday ... the day before she filed separation papers.

The source tells X17online:

    "Heidi and Spencer share a bank account, and it's no secret that Spencer has been spending her money like crazy. She obviously knows that she makes way more money than he does, and he knows that, too. People think this separation is fake, but it's 100% real because she knows Spencer is ripping her off. The divorce definitely should be happening by the end of the week or by early next week, but the separation had to happen first until she gets her finances squared away."

We've learned that Heidi's suspicions go way further than just Spencer's hefty spending habits. Our source confirms that Heidi believes another girl has been spending time with Spencer since they've separated. The source adds:

    "To make it worse, Heidi has her suspicions that Spencer is secretly spending her money on another girl. Her accountant has recently notified her of some big purchases from a jewelry store. When she confronted Spencer, he said he bought himself a new watch. Heidi doesn't believe him. That's why she hired the PI, and after a week of following him, Heidi's convinced that another bitch is getting showered with gifts at Heidi's expense. She's over it."

Spencer has no idea that Heidi has hired a PI to investigate him, says our source, "but it looks like he's going to find out now."

Rumors of Heidi shacking up with Spencer's best friend, Cougar are somewhat true. X17online can confirm that Heidi is living with Cougar Zank in Malibu. As for a relationship between the two, our source says "it's a possibility." Spencer and Cougar have recently had a falling out because Cougar is now "siding with Heidi." Mr. Pratt claims he has reasons to believe that Heidi and Cougar have been cheating behind his back.

As for where their relationship stands now, "Spencer has definitely been trying to win Heidi back." Just last Thursday, he paid a visit to her Malibu home. "Spencer didn't actually think that Heidi would go through with filing for a separation, so he was definitely shocked," adds our source. Whether or not these two will get back together, no one is 100% sure, "but they're definitely a married couple going through problems."

A rep for Heidi and Spencer tells us "no comment."