I am running a model with which I want to explain students' performances. I lagged my main independent variable by two years. Now, my results show a negative coefficient for the two years lagged independent variable. However, when I add a higher lag of that IV - let's say a three or 4 years lag - the coeeficient of the two years lagged IV becomes postive while the higher lags are negative.

Is that normal? How do I interprete these results?

Could it be that my highest lag (in this case lag2) also represents higher lags?

Sorry for any wordiness and thank you very much for your help.