Please help!!!! Weird spotting

I need help with an issue. We have been ttc since October, I skipped my period in November and got a negative pregnant test. Then around the 20th and 21st of this month I began spotting as if my period was coming. The spotting lasted 3-4 days. It was so light I literally used one panty liner a day. My normal period bleeding never came. It started out that brownish color. Then on Christmas <a href="">Eve</a> me and my husband had sex and I expected it to " knock my period out " like it usually does. But nothing, it did turn red for one day instead of brown. The spotting ended on Christmas so I too a test and again negative. I'm not the kind to stress out over this, were not tacking or doing a lot to ttc, just taking it a day at a time, but this is boggling my mind. I just wonder what the weird spotting was.