Confused..

Jennifer • 35 years old TTC.
My husband and I are TTC.  We started being active towards the end of Feb and beginning of March. We started last year but we weren't as active. Here is my chart below.  The last time we had sex was on March 5th.  Then on the 11th I had light spotting. Lasted for a day and a half. Glow said I would start my period on the 20th which is today..and of course it's here. :( I use to be irregular. Seems like now I'm regular.  Question is..why was I spotting?