I think an old FitBit is somehow paired with my Apple Watch

Last night I found an old FitBit that I had discovered my sister wanted. I thought I'd surprise her and give her mine since it had been in a drawer for at least 6-7 years. When I tried to charge it I could feel it vibrate but nothing showed on the screen. So I decided to leave it plugged into its USB charger overnight. This morning I just woke up to look at the time on my iWatch and saw a totally different face! OMG! Looks like the FB face maybe? But then it jumps to the iWatch face and back to waiting for me to put in the security code - which I have NOT done. I'm now looking at the FB watch which still has a black face but when I tap it, a green light comes on the back, vibrating. What have I done?! And all I wanted to do was surprise my sister with the FB and certainly not mess up my watch. I have no clue where to even start. My 41mm iWatch is an 8 (GPS) v11.5. I don't remember what the FB was - probably 3?

[Re-Titled by Moderator]

Original Title: I think an old FitBit is somehow paired with my iWatch