First of all, let me say that this is completely normal and you have nothing to be worried about. Almost all women go through spotting. While some women have spotting issues regularly, most women get it just one in a while. Menstrual fluid is made up of blood as well as cervical mucus and vaginal secretions. If you have an extremely light flow, it would result in a little blood seeping out along with your normal vaginal secretions and this causes the brownish stains that you see. As your flow is light, the blood remains within you for longer which causes it to becomes more brown than red. If you are trying for a baby, then you need to consider this as the first day of your period when making any calculations.
If this spotting always appears a day or two before your actual period, you can use a panty liner to avoid staining your clothes. However, make sure that you only use a panty liner if your menstrual cycle seems to follow this pattern regularly. Very often women try to substitute panty liners for pads with disastrous results. A liner is only meant for spotting and cannot withstand a regular normal flow.
answered by M W