Weirdest thing ever

Hi everybody

I lost some nice inches on mfp lately and overall pretty happy how my body changed.
Except this one small thing. Note: I almost ditched cardio, instead I walk to and from gym every time (like 20mins)
Yesterday out of boredom and free time I went for a short run on speed 9 treadmill.

Guess what I felt like my butt was flying up and down and felt no control after it. It made me feel so uncomfortable (partly coz I got scared somebody watching me in tight pants and this shaking)

It wasn't like fatty bitty shake, more like whole butt moving from the bone .. Ahm I dunno how to explain better

Any opinions? I m 5.3, 126 lb, 25% fat