"I've hit a lot of drivers, you put a lot of force on your body when you do that," he told reporters at the South African Open
Rory McIlroy didn't sleep well last night.
Back pain forced him out of bed to take something to ease his back pain and he went out today to shoot a third round 67.
Crowds at the South African Open in Johannesburg were relieved to see him driving well and attacking the course from the off.
"It was a little tender this morning," he told reporters this afternoon. "I didn't really get much sleep last night, couldn't get comfortable.
"The physios did a good job on me this morning and strapped me up pretty well."
His work on the greens showed that perhaps he wasn't feeling the squeeze as much as he says, with two birdies and an eagle to show for a good front nine, with only a bogey round the turn the solitary blemish on his card.
Consecutive birdies from 11 to 13 was followed by four pars and a bogey finish.
"[Physios] gave me a couple of anti-inflammatories which helped. I was uncomfortable for the first five or six holes but then, I took another few anti-inflammatories in the middle of the round and it felt pretty good on the back nine.
"Hopefully it continues to feel that way and that the muscle spasm I have around the joint is subsiding. I'll hopefully get some treatment today and tomorrow before I go out."
There was a risk that aggravating an injury this early in the season could take it's toll on him later in the season, but McIlroy offered a suggestion as to why he had been suffering.
"If it's a muscular thing, it's fatigue. I've hit a lot of golf balls over the three or four weeks testing equipment. I've hit a lot of drivers. You put a lot of force on your body when you do that. I'll just take it easy over the next few days and with some rest and treatment I should be fine."
Discussing his round this afternoon he said: "I'd like to be a few shots better or at least a few shots closer to the lead. Hopefully I can get off to a good start tomorrow. Put a bit of pressure on Graeme [Storm, leader], it'll be interesting."
McIlroy sits outright second, one stroke clear of Jbe Kruger and Jordan Smith and three strokes behind England's Storm.