My rat is behaving strangely...?

So my rat and I get along very well. I've had him since he was about a month old and he's quite big now. We have had no real problems in the past, I mean when we were play fighting as we always do he bit me and made me bleed a couple weeks ago but I chalked that up to me being too rough with him. But twice now he has freaked out if I touch his back, sides or neck. Then he will turn to his stomach and he won't mind me touching him there. The first time I did it I just thought he was playing so I carried on tickling his sides but he was screaming so I stopped. Then I saw that he was breathing really heavily and I didn't know what to do. He calmed down after a few minutes and I could touch him again. The same thing happened again today but when he started doing it I tried to look and see if there was any specific spots that he didn't want me to touch then I left him alone. There was no specific spots, just his whole back. Then he flips onto his belly like he's trying to protect his back. Why does he do this and is there anything I should be concerned about? He's literally fine now so I have no idea why he just started freaking out. Any help is welcome.