。
Aren't you studying too much?
Literal
Study too-much [tag question-んじゃないの]?
勉強のし過ぎ nominalizes する via [noun]+の+し過ぎ ('too much of X-ing'). The し is the stem of する. The ending んじゃないの is a casual tag-question, softening the assertion into a concerned question. This is friendly mother/friend speak — 'aren't you overdoing it?'