Can a lipoma show in an ultrasound but not MRI? Can a small one in calf cause pain and twitching of toes?

6 months ago, my toes started twitching like crazy. It only occurred on my left foot and wasn't painful, so I ignored it for awhile. Since then, my issues have progressed. The twitching does come and go, but my toes will sometimes lock in a downward position and require me to physically pull them back up. Cramps in calf too. AIl are quite painful. Recently I felt a lump in my left calf. Not huge, but there. Ultrasound revealed "lipoma" or another small mass. Had MRI...came back normal?