B737-800
Inflight Event / Encounter Wake Vortex Encounter
Flight Crew Took Evasive Action
Procedure; Environment - Non Weather Related
Descending on left downwind for RWY 27L at ORD from 7000 feet to 5000 feet with the autopilot engaged; at about 5500 feet during the descent we encountered a severe rolling moment to the right along with moderate turbulence. The FO was flying and disconnected the autopilot and after a moment we seemed to have flown out of it.  But a second moderate turbulence event along with another severe rolling moment was encountered.  We reported to ATC that we were having difficulty in controlling the aircraft altitude because of the severe rolling moment. The FO leveled the aircraft and began a slow climb back to approximately 5500 feet and after another moment we seemed to be out of it once again.  We asked the controller who we were following because we just felt like we encountered wake turbulence and the controller said we were following a regional jet on final.  Following the encounter and after a brief exchange with the ORD ground controller; we believe that we may have encountered wake turbulence from a heavy jet on approach to RWY 28C.  However we are not clear on the proximity of that aircraft to us.
B737-800 Captain reported encountering wake turbulence on approach to ORD; possibly from a preceding heavy jet.