Kristen - who was romantically involved with Rob while filming the first rental of the Twilight film franchise - discovered she could be expected last week.
And Stewart, 19, was so worried that someone would discover her secret, she sent a close friend to buy a pregnancy test kit.
"Kristen asked one of her friends to buy it because she was concerned for some time to see her at the drug store," a source told Australian magazine NW. She is very nervous about the whole situation. "
Kristen - who plays rocker Joan Jett in the new movie The Runaway - began to suspect she could expect when she realized she had not had her period.
"When she went she was too late, she obviously started to believe that she could be pregnant," says a pal. "And because of the timing, she thinks Rob could be the father."
The alleged pregnancy could be why Rob, 23, has been so stressed lately.
The actor is currently in New York, filming his new flick "Remember me.
And sources debut in the film are obviously concerned because Pattinson has lost so much weight.
"He is so thin and stressed," a source told New York Daily News newspaper. "All sets are concerned about him.
"Robert does not understand why everyone is so crazy over him.
"He is embarrassed by the way the girls throw themselves at him. The girls here are stalking him.
He stayed in two different hotels during the course of four days, only to try to flee from fans who were after him.
"He is afraid that if he gives a hand, they take the whole arm. He is informed of the security not to encourage the public, so that he does not even look up anymore."
