The right-hander grimaced after throwing a changeup to Brian McCann, the sixth batter he faced, with one out in the second inning Friday night.
Strasburg retired McCann on a grounder and struck out Dan Uggla to end the inning.
The Nationals did not immediately announce why Strasburg’s night ended so soon. He threw 23 of 37 pitches for strikes.
Washington shut down Strasburg on Sept. 7 last season to limit the workload on his surgically repaired arm, keeping him out of the NL playoffs. The top pick in the 2009 amateur draft, he underwent elbow ligament-replacement surgery on Sept. 10, 2010.
The 24-year-old Strasburg was making his 12th start of the season. He began the night 3-5 with a 2.49 ERA and 71 strikeouts in 72 1-3 innings.
After getting off to a slow start this year, Strasburg was 2-0 with a 1.17 ERA in his previous three starts.