Help Vitex question

When taking Vitex do you have to wait for your period to start or can you take it as soon as the bottle gets here ? I'm taking this due to my period being only three days and two days being brown blood (STMI) the doctor said I need my periods to be a nice red blood to have night chance of getting pregnant..note I was on Mirena 4 years no periods I have been of it since 9/11/15 and my periods have been on track 28 days but brown blood or black red cloths so I want to try Vitex