I am guessing here but maybe the Pharmacist thought you were trying to pull a fast one by getting two months meds in one months which is not allowed under the DPS scheme.
However, as the presciption is for 28 days not a calender month, you should not have paid. You will have to get this presciption filled 13 times each year, you should only be paying 120 * 12. You are not getting a month free.
I would suggest explaining this to the Pharmacist and insist on my money back,
Alternatively, contact the DPS people directly, explain the situation, and ask for a refund.