Beltran says he felt the problem on Sunday.
Beltran has been limited to designated hitter duty since coming off the disabled list June 5 after missing 21 games with a bone spur in his elbow. He played catch for the first time June 11 and has been working since then on rebuilding his arm strength with two sessions every three days.
''The throwing program was going well until that day,'' Beltran said Tuesday. ''I was letting it go, I felt good about it. The next day I got to the ballpark and my forearm was real tight, like there was a knot in there.''
Beltran is batting .220 with seven home runs and 23 RBIs, and feels no pain when swinging the bat. He expects to have surgery after the season to remove a loose chip.