I'm guessing either a fungus, or sun scalding. Or maybe blight.
With a fungus, it should also affect the leaves, however. A treatment of neem oil should help this.
If the plant got wet and the sun was very hot, the remaining water can burn the petals and you will get smooth, almost round, brown spots.
In blight, petals turn brown and fall. Spots may form on the leaves, especially where faded petals have fallen, and flower buds may be killed before opening. This can be caused by the plants being too close together, not permitting enough air circulation. I would also not put any cuttings/trimmings in the compost bin - throw them out.