Aileen, that's a trifle speculative......
So nobody else on here is being speculative? As I've already said it's lack of information qualifying these announcements that gives rise to speculation. If the injury was picked up in Barcelona then, in fairness to his fans, why not say so?
Yes, I think somthing did happen in Barcelona rather tentative a few times reaching for a ball. Good thing he pulled out of Madrid because I believe it is a very slippery surface. a few players may tumble. He will be fine for Rome and RG.
I noticed him clutch his back in the Raonic match, but sometimes with Andy you can't be sure because he often clutches parts of his body when the pressure's on.
If it is his back, then the fact that he was apparently able to practise in Madrid suggests it could be a muscle strain which, very sensibly, he doesn't want to aggravate, something that could be likely on this surface, which Nadal has described in the article linked below as "soft and slippery".
Fortunately the ATP were granted the use of this surface for only one year "with the understanding that it will be reviewed following the event, of course taking into account feedback from players."http://www.sportsnet.ca/tennis/2012/05/04/madrid_open_rafael_nadal_blames_atp_for_allowing_blue_clay/