Contreras hit a grounder to Reds shortstop Paul Janish, who bobbled the ball before picking it up and making the throw. Immediately after hitting the bag, Contreras pulled up and received attention from the club's training staff before walking off the field.
The status of Contreras' next scheduled start is unknown. Ubaldo Jimenez (13-10, 3.32 ERA), who is nursing a left hamstring strain, has been moved into the spot that Contreras (1-0, 1.86) would normally occupy -- Tuesday on the road against the Giants.
The Rockies will check out Contreras on Friday.
"It didn't hurt that bad ... more like a cramp," Contreras said in Spanish, with Rockies assistant equipment manager Joe Diaz translating.
After Contreras left, Colorado scored five runs off Reds starter Kip Wells to take a 5-1 lead.
Contreras gave up four hits but managed to hold the Reds to one run in his three innings. He struck out five and walked three.
Right-hander Jhoulys Chacin, 21, the Rockies' top right-handed pitching prospect, replaced Contreras to open the fourth inning. He battled command issues, walked one and struck out one before being replaced by Juan Rincon.
Colorado manager Jim Tracy said the pitcher who will replace Contreras will not be Chacin, who is in the learning stages and needs to master fastball command before he can start for a team chasing a playoff spot.
The Rockies acquired Contreras in a trade with the White Sox on Aug. 31. He won his debut, pitching 6 2/3 innings and giving up one run on eight hits in a 4-1 victory over the D-backs on Saturday night.
Contreras is with Colorado because it needed a seasoned starter when Aaron Cook suffered right shoulder soreness during a start on Aug. 21. Cook has not pitched since, although he is slated to throw a bullpen session on Friday.
Thomas Harding is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.