Hate to say this -- but you are at 38 days now, and I would dare guess thay she didn't show you a heat at her first return, just a silent type thing. Today she is showing you by signs of discharge that she is 24 hours or so away from being in heat for real.
Doubtfull she had an early embryonic death seeing how the days are close to normal. She could be a 19-20 day cow -- and no one seen her at her first cycle. Sorry -- but every few years or so I get one thaty does it to me too, last year we did have one I simply missed.
It takes alot of practice to get good at reading cow language, this is chapter 2 of "I'm in heat 101" hehehe.
Watch her real close, and I'd bet that you will see sometihng going on abnormal in a little while. Meybe you won't see her standing in heat, but the bull will know -- she is talking to him now!!
Good luck -- no worry -- just I think she is bashfull (Ok -- I'll stop there)
Terry