Weight fluctuation problems

so i’ve had random fluctuations for a long time. and i’m noticing a bit of a trend. these fluctuations start and end with no change from me each time. i randomly went from 180 lbs to 135 in 8 months. and that weight lasted for one year and now i have gained 24 lbs back with no change in diet or exercise.

WHY does this keep happening?? in the past i’ve spent a crazy amount of time at the gym and not eating out of control and that never got rid of the weight. is there something i’m missing or something i should ask my doctor about or should i see a nutritionist and try extremely strict diet? i’ve dieted, but they never seemed that difficult to follow. which is good i guess? but i don’t know what would actually work because i really don’t want to go through this again.