If I get on the scale 3 times in 3 minutes I get three different numbers. why?

Im trying to lose weight I started off 149 3 mths ago now Im around 135 but I want an accurate reading!! The scale went from 139 to 137 to 135. (With shoes on) Ugh!! Is something wrong with my scale? Cuz I paid 35 dollars 4 it. Or is it just this old house. Im 18 and I've been living here all my life. The floors maybe unbalanced or something. I don’t kno. I just want an accurate reading. Someone HELP!!