confused by fertility test


so I've been using first response fertility tests. I'm testing twice a day the same time every day. Each day my test line a got a little darker. yesterday I tested and the test line was dark but not as dark as the control line. I tested today and my test line is barely even showing up. what gives? is it possible that yesterday was my lh surge and the test didn't just get as dark? doesn't seem like it would just get lighter for no reason. I'm not on any medication or anything. I'll include yesterdays and one of today's test