@ysmina-- I'm not sure but I'm guessing it's from friction that happened during exercise. Were you wearing really tight clothing? You might want to wear loose clothing for a while and just keep the area clean. Don't pop it. Hopefully it will go away soon. And if you want to prevent more blisters, try to keep yourself dry while exercising. I usually keep a small towel with me and wipe my sweat at the gym.
If it doesn't go away or if more show up, you should probably see a doctor in case it's caused by a bacteria or a virus.
The only time I got a blister around my belly button was in the summer and an insect bit me while I was outside. I had a minor allergic reaction to it and got a blister there. It went away after applying some corticosteroid cream though.