Can long term use of Vitex (Chasteberry) cause very light periods?

I have 10 day long periods, with maybe two days of light to medium bleeding and the rest of the time spotting. This isn't normal for me, but they've been that way a long time now. I've been on Vitex over a year. Could that be the cause, or do I need to see the Dr? I'm afraid that even if I conceived, there isn't enough nourishing blood layer on the inside of my uterus for the fertilized egg to implant itself to make it a viable pregnancy.