Harrison, who missed virtually all of last season after having surgery to repair a herniated disk, started this season on the disabled list. He has made only four starts this year.
"Lower back more on the left side than the right on the left. It was affecting me driving through the baseball," Harrison said. "It felt like all arm. I don't really know why it happened what caused it or what I did wrong. Hopefully, I get good news and go from there."
Harrison struggled from the outset against Houston, allowing four hits and walking three batters on a cool evening -- the roof was open -- at Minute Maid Park before leaving the game.
Harrison, who said he's driving home with his wife in the morning, said he will be evaluated by back specialist Dr. Drew Dossett in Dallas on Wednesday afternoon.
"Frustration to begin with and then thinking about what happened last year and all I went through to get back. I really don't want to have to go through that again," Harrison said. "I'll find out definitely tomorrow what's going on and get my head wrapped around that. I really don't want to have that issue again. I'm just praying for that."
Harrison said he doesn't remember doing anything to aggravate his back.
"Something like that comes out of nowhere and just hits you," Harrison said. "It's not really just one thing that I felt differently. Last year it got progressively worse but I don't remember the one thing that caused it."
Harrison's fastball registered between 86 and 88 mph much of the night. His average velocity this season according to FanGraphs.com has been 90.1 mph.
Harrison walked two in the first inning, bringing pitching coach Mike Maddux to the mound for a visit. In the second inning, he gave up a single and homer to L.J. Hoes to start the inning.
After getting two outs, he allowed a double, a walk and a single to Dexter Fowler, driving in a run and bringing manager Ron Washington, Maddux and the club's trainer to the mound.
On Monday, Harrison talked about the importance of pitching deep into games. The Rangers have only 13 quality starts, games in which the starter goes six innings and allows three runs or fewer.
Only Baltimore and Tampa Bay have fewer.
"I want to pitch deeper in the game and give the bullpen a break," Harrison said. "I need to limit those walks, get ahead of guys more often and minimize those runs at the beginning of games. That's the only way you get deep into games."