This could be down to two things. Please have a read below and see which one sounds like you.
Recently changed your repayment day? Your next monthly repayment will be slightly higher or lower than usual. This is to adjust for the change in interest payable caused by the change in number of days from your original repayment day to the new day. It's OK, this is a one-off! Your repayments will return to normal the following month.
Recently paid extra towards your loan? This will reduce your monthly repayment amount. (The term of your loan will stay the same.)