Spotting... help, opinions?

Ok so we have been trying for 6 months. My periods are regular with a regular flow and this month it showed up on time. I started spotting on Saturday and that's all it has done it spot. I've been spotting for four days...this is so so weird for me and I don't know what to think...any suggestions? I last tested Saturday morning (before the spotting started) and t was negative at that point...suggestions?