Strasburg (3-5) was chased after giving up six runs and seven hits in 3 2/3 innings of Washington's 8-1 loss to the Philadelphia Phillies on Saturday that snapped the Nationals' six-game winning streak.
Over his last three starts, Strasburg has allowed 15 runs in 12 innings.
"Yeah, it's frustrating. I'm not pitching to my ability, but I've got to keep grinding," Strasburg said after seeing his ERA climb to 6.50.
"It's definitely something I've never experienced before," he said of his recent struggles. "This is a test for me."
Strasburg struck out three and walked one. He trailed 2-0 with two outs in the second when Maikel Franco hit a two-run homer on an 0-2 pitch.
"He had a lot of hitters at two strikes and couldn't put them away," Washington manager Matt Williams said. "He's going through a rough stretch. That doesn't mean he can't come out of it and be absolutely dominant. He has the capability every time he walks on the mound."
Hamels (5-3) won his fourth straight start, and has compiled a 1.53 ERA over that stretch. He gave up one run and five hits, striking out five and walking one.
Franco finished with two hits for Philadelphia.
Hamels, who had a shutout going until giving up Ian Desmond's RBI double in the eighth, snapped his six-game winless streak against the Nationals.
Washington's Bryce Harper went 0 for 4 and slammed his bat in disgust after striking out in the ninth inning.
The Phillies got to Strasburg for four runs in the third. A one-out double by Cesar Hernandez put runners on second and third. After Chase Utley's grounder scored Ben Revere, Howard lined a double down the right-field line, sending Hernandez home and bringing Franco to the plate.
Three Washington errors contributed to Strasburg's exit in the fourth when the Phillies added two runs.
Nationals: INF Anthony Rendon (oblique, left knee) took full batting practice on the field for the first time. Manager Matt Williams said Rendon will play simulated games in Viera, Florida, beginning early next week. ... Williams said OF Jayson Werth (left wrist contusion) still has swelling in the wrist and is unable to swing a bat.
Phillies: RHP Aaron Harang (4-3, 1.82) hasn't allowed an earned run over 14 innings in his last two starts. He's 4-5 with a 3.16 ERA in 14 career starts against Washington.
Nationals: RHP Gio Gonzalez (3-2, 4.94) is coming off two no-decisions in which he's allowed 11 earned runs in 10 innings. He's 6-5 with a 3.15 ERA in 13 starts against the Phillies.
Washington RHP Tanner Roark makes his first 2015 start Monday in place of RHP Doug Fister (forearm tightness), who's on the disabled list. Roark was 15-10 with a 2.85 ERA as a starter in 2014, but has pitched in relief this season. RHP Jordan Zimmermann will start Tuesday with an extra day of rest.