My rats racing round cage, why?

Hello folks. Recently one of my pet rats Milky has started taking mad episodes of bolting round his cage for no apparently reason. We thought it might have been to do with his cage mate but yesterday Milky had to have a small operation and so needed to be seperated to recover and he took an episode in the evening. Its very hard to get him to stop and we have to shout and bang the cage to try distract him to stop. Hes broken some more nails and they were bleeding and took him ages, longer than normal, to recover from this one. Im really worried it might be seizures or something along that lines. My other rat did it once when i had the hoover on but that that never bothered Milky but hes started doing it for no reason that we can see. Naturally i spoke to my vet about this and she gave him a health check and said his heart n lungs were all ok and it might just be a behavioural thing. If anyone can offer any help,advice,information id be emensly grateful. Thanks for now