calculate movement based on previous row

here is some sample data from the orders table. sql server 2019

month/year  orderamount
jan-2021      200
feb-2021.     0
mar-2021.    250
Apr-2021.    300
May-2021     100

trying to write a query that will take into account the order amount of the current month and the previous month and put result in a column called movement

need to use following criteria

if it is the first month or last month column should be zero

if given month value is not  > 0 then 0

if given month value greater than 0 and next month greater than given month need the difference

if given month value greater than 0 and next month less than given month should be zero

given sample data above

query results should be

month/year  orderamount  movement
jan-2021      200                  0
feb-2021.     0                       0
mar-2021.    250                  0
Apr-2021.    300                  50
May-2021     100                 0

would this be good use of lag function ?