ANSWER:

Yes, you are sinful for being like this in a place where there are CCTV cameras and men can walk in like this! 

QUESTION: Sheikh I was working out in an only women gym so I wasn’t wearing my hijab, at some point a random grown man opened the door and was about to walk in because he confused the place for something else. As soon as I saw him I hid behind a wall to block his view but he had already seen me in a tshirt, leggings and no hijab.. am i sinful